To the Fun Science Gallery Contents

 

2 – Small Terrestrial Animals

Giorgio Carboni, February 2007
Translated by Sarah Pogue

 


 

CONTENTS
 
INTRODUCTION
FREE OBSERVATIONS
Aphids
Ants
Bees and Wasps
Butterflies and caterpillars
Spiders
Let’s throw away the old clothes!
Mint is so good!
Flies and Hoverflies
Dragonflies
Insects on flowers and leaves
Praying mantis
Photophobic animals
Plant parasites and diseases
A lesson on the insects
Don’t stress the animals!
 
 
INFORMATION ON THE INSECTS
The body
The legs
The wings
Reproduction and development
Behaviour
Classification
IN DEPTH
CONCLUSION
BIBLIOGRAPHY
INTERNET RESOURCES

Figure 1 – Praying Mantis.
(In a flowering field there is always
someone that observes us!)


INTRODUCTION

In this article we will deal with small terrestrial animals. These include some small terrestrial Molluscs (snails) and many Arthropods (Insects, Spiders, Myriapods). With one exception (woodlice), we will not deal with Crustaceans because, although they are Arthropods, they are essentially aquatic organisms.

The number of classified species of Insects and other Arthropods is over one million, but the actual number of species in existence is estimated to exceed 50 million. It is obvious that also in this case I will limit myself to an introduction, leaving to the readers the joy of exploring the environment and searching out small animals to observe under the microscope.

The Arthropods are animals with jointed limbs. They descend from ancestors similar to the Annelids and they have a segmented body. Another important characteristic of the Arthropods is the exoskeleton, a true suit of armour the supports and protects the animal. The Myriapods are Arthropods in which the segmentation of the body is still clearly visible and which possess a large number of legs such as millipedes and centipedes. The Arachnids, in general, possess 8 legs, a pair of chelicerae and a pair of pedipalps. The spiders, scorpions, ticks and mites belong to this class. The Insects have 6 legs and, comprising more than 70% of all species, is the most numerous class of the Arthropods. Amongst the Invertebrates they are the only animals capable of flight. Their bodies are divided into the head, thorax and abdomen, they have a pair of antennae and the majority are equipped with one or two pairs of wings. The class of the Insects has been subdivided into 29 orders, of which the most numerous are the Diptera, Lepidoptera, Hymenoptera, Coleoptera, Hemiptera and Orthoptera. The insects have an important ecological role as predators, prey, parasites, detritivores etc. They are also very important due to their role as pollinators and as producers of honey, wax and silk. Finally, they are important in the fight against harmful species.


FREE OBSERVATIONS

It’s spring, the rainy days have finally come to an end and today the sun is shining. It is the perfect moment to go into to the garden or to a field to carry out microscope observations. In the vast majority of cases, it will be necessary to use a stereoscopic microscope. If you have to buy it, get a model whose minimum magnification is 10 X. Be aware that 20 X is too high for a minimum magnification. If it is an instrument with two magnifications, a model with 10 and 30 X is preferable to one with 20 and 40 X. If possible, purchase a microscope with continuous variation of magnification (zoom) and which has eyepieces 30 mm in diameter – this will cost you a bit more, but I assure you that you will always be happy with your choice. Don’t be afraid to spend a little more for a microscope. It is an instrument that will last you for all of your life and that will also be used by your children and grandchildren.

APHIDS
Seen under a stereoscopic microscope, a rosebud infested with aphids is an incredible spectacle. With this instrument, it will be easy to observe the winged adults, others without wings (apterous) and the young of the aphids which seem miniature versions of the adults. You can see a mother aphid giving birth, aphids that oscillate in order to better drive their rostrum into the tissues of the plant and aphids which are moulting, abandoning the white exuvia. You can also observe parasitic aphids that have assumed a strange fixity and a colour similar to that of terracotta.

 

Figure 2 – Winged aphids.

Figure 3 - Aphids.

Figure 4 – Aphids during the moult.

If you remove the shoot of a plant, after only a few minutes you will find aphids everywhere as the flow of sap has been interrupted. In order to avoid this inconvenience, you can leave the shoot in place and position the microscope above it. To do this, you can fix the microscope onto a tube which is then driven into the soil near the plant.

While they suck the sap of the plant, the aphids emit drops of honeydew, a sugary liquid on which ants feed. Leaving the shoot in place, it will also be possibile to see the ants as they use their antennae to stimulate the aphids to produce the drops of honeydew which they then suck avidly. The ants protect the aphids and also move them to other plants as thought they were raising them.

If ladybirds, green lacewings, or their voracious larvae are present on the shoot, you can observe the horrible spectacle of the aphids being eaten alive a little at a time. Strangely, as you observe a larva eating the posterior part of the insect, you will realise that the insect continues to do what it was doing before without displaying any signs of suffering for what is happening to it.

ANTS
If you capture an ant and you place it in a Petri dish, you will not be able to observe it calmly because it runs tirelessly from one side to the other trying to escape. If, instead, you put it in a dish with a drop of honey mixed with the same quantity of water, the ant will stop to feed and will remain still for a long time, allowing you to observe it and also to take photographs.

Figure 5 – An ant intent on feeding on honey.

Figure 6 – Digger wasp.

Figure 7 - Wasp.

 

The most natural way to observe ants is to leave them in their environment. You can fix the stereoscopic microscope beside an ant trail, or you can observe a group of ants as they transport a piece of food to the nest. One of the most fascinating spectacles is the observation of a colony. You can see the ants that exit from the tunnels carrying out a grain of soil. They often deposit it just outside the mouth of the tunnel, so that the soil rolls back in. At other times, you see the ant climb up a tall blade of grass and drops its grain of soil from there and this also rolls back into the tunnel. In spite of these failures, other ants succeed in carrying their soil outside definitively so that, with time, you see a small mound form.

A group of ants intent on dragging a live animal into the colony is quite a disturbing spectacle, but this never arrives at the ferocity of a war between ants. We often imagine that ants are hardworking pacific animals, but they also have their battles. Walking along a footpath, you could happen to come across a dark mark which at first seems to be motor oil. If you move closer, you will instead see a swarm of ants, but it will be difficult to understand what is happening. If you have a microscope with you or a reasonably powerful lens, you will see that the ants are fighting ferociously amongst themselves. You will see ants that bite the legs of their adversaries and others that with their jaws have pierced the stomach or skull. Some of those fighting will have a large head and these are the soldier ants, but the battle will also be fought by the workers. At the end of the war, the ants left alive carry away the bodies. After some time, the dark mark will disappear without leaving a trace. The passerby that saw this mark won’t even remember or suspect in any way that a bloody battle was fought in that spot leaving thousands of victims. Nobody will know who won and no history book will mention it…. unless there is someone else, apart from ourselves, that observes our battles.

BEES AND WASPS
Using a telescope, you can observe bees feeding on nectar and collecting pollen without disturbing them. With a Petri dish you can easily capture one to examine it under the stereoscopic microscope. To make it stay still, offer it a drop of honey. Normally, you will see a large number of pollen grains on its body entrapped in the hairs. On the posterior legs, bees possess a basket, formed from curved hairs, which is used to collect pollen.

The bumblebees are Apidae. Examining them attentively, you can often find parasites on their bodies. If you want to liberate them from this annoyance, you will have to immobilise the bumblebee with cotton and, paying attention to avoid getting stung, you can remove some of the parasites and see them under the microscope.

Wasps are also beautiful to observe under the microscope. Their yellow and black stripes warn you of the danger of the animal. Their sudden movements and attentive attitude make you understand that you have in front of you an insect equipped with a notable intelligence. In fact, differently to the flies, wasps do not fly into windows, or they do it only once, after which they carefully search for the exit. If a wasp enters your house, don’t kill it, but with a newspaper accompany it towards an open window or door. With the help of a telescope you can observe the intense work in a wasps nest.

The majority of bees and wasps are solitary. The digger wasps have slender and rapid movements (Figure 6). Of solitary character, they make their nests in sandy soils. Some species carry spiders that they have paralysed with their sting there. After depositing them on top of an egg, they close the nest and go in search of other spiders. Other species, instead, use caterpillars as a food reserve for their larvae.

BUTTERFLIES AND CATERPILLARS
Capturing a butterfly in flight is always rather difficult, unless you have a net specifically for this purpose. At dusk, it is possible to find butterflies resting on blades of grass or on flowers, ready to pass the night there. Butterflies are very delicate, therefore, if you want to observe the wings, try observing them in the field, otherwise try keeping them in a Petri dish. Of a butterfly, observe the scales on the wings and the rest of the body, the compound eyes, the antennae and the proboscis (sucking apparatus in the form of a long tube which, when at rest, the insect keeps coiled under the head). As you well know, the butterflies have beautiful patterns and colours and the photographs that you will take of these insects will have a certain effect. Try to see the difference in the shape of the scales of butterflies, moths and mosquitoes.

As you know, butterflies pass the first part of their lives as caterpillars. These pleasant animals have difficult tastes, so much so that they often feed upon the leaves of a single plant species. Between spring and summer it is possible to note the plants whose leaves have been eaten at the edges. If you are lucky, you can observe the caterpillar as it devours the leaf with subsequent "droppings" on the edge of the leaf. Otherwise, search for the caterpillars on the underside of the leaves with a mirror. With suitable containers and supplying the caterpillars with the right plants, it is possible to raise butterflies. Once, while I was observing a caterpillar with a stereoscopic microscope, I noted some grains on the microscope base and on the table. After a few minutes, these grains increased in number. I didn’t understand what was happening, until I saw the caterpillar raise its posterior and shoot excrement in the form of pellets a certain distance. I called my wife who began to laugh watching this funny behaviour. With this I wish to say that during microscope observations completely unexpected things can happen.

SPIDERS
One day I was observing a flower crab spider (genus misumena or similar). It was an animal 7 - 8 mm in length, yellow-green in colour and which very much resembled a crab. While I observed how it raised the anterior legs as a wisp of straw moved towards it, I realised that in the head behind its eyes two shadows moved. I could distinguish two dark segments inside the head parallel to the two anterior eyes. Upon closer observation, I realised that those two shadows were none other than the retinas of the eyes and that the spider moved them to direct its gaze. Spiders have 8 eyes. These are not compound and faceted like those of insects, but each is formed of a hemispherical lens. In the flower crab spiders, the two anterior eyes are generally larger than the others. In contrast to our eyes which can rotate in all directions, those of the spiders are fixed, therefore in order to direct their gaze these animals can do nothing other than move the retina.

Another time, I captured a jumping spider. These spiders live upon rocks, on the external walls of houses and in other similar environments. This animal was approximately 10 mm in length, the cephalothorax was black, while the ventral surface was red. It was probably a male of the species Philaeus chrysops. The legs, also black in colour, were short and robust. I put the spider in a Petri dish and, after observing it well from above, I turned the dish upside-down to observe the ventral surface. Rather than fall, the spider remained well-anchored to the cover of the dish and I could observe its underside. The first thing that I did was to look for suction pads on the legs, but there weren’t any. I searched for the claws, but I didn’t see those either. So how did it remain attached? To my surprise, I realised that at the extremities of the legs there was a kind of brush whose numerous hairs terminated in very fine filaments. Therefore, I immediately thought of the geckos, animals similar to lizards which live on the walls of Mediterranean houses. These animals also have “brushes” on their feet and, in accordance with biology, the bristles finish in extremely fine filaments that adhere to the wall by means of electrostatic atomic bonds called van der Waals forces. Many insects also have adhering structures such as this at the end of their legs.

In another case, it was again a jumping spider. Its grey body was approximately 8 mm long, the legs were short and robust. The most striking features of this animal were its two large black frontal eyes. Seen under the stereomicroscope, it was like looking at a jeep. To make the animal move, I moved a finger near to it and I realised that it was mirrored in its eyes. Looking more closely, I noted that the eyes of the spider reflected my entire image. Well, I had an enormous finger attached to a long thin arm, while my body was very far away. That spider had a giant in its eyes and it was terrified. In that moment, I literally saw myself from a perspective that I had never thought of before!

LET’S THROW AWAY THE OLD CLOTHES!
The growth of the Arthropods is limited by the exoskeleton. In order to grow, these animals have to periodically rid themselves of the old skin or exuvia. Looking carefully in the grass, you can come across a spider exuvia. It is extremely light and delicate. Observing it under the stereomicroscope, you can see the corneas of the eyes and the area of attachment of the legs. It is a bit of a horrible spectacle, but it’s worth it! It is also easy to find the exuviae of grasshoppers. On pond reeds, it is possible to find the exuviae of dragonfly larvae that have abandoned the aquatic life and embraced the air.

 

Figure 8 – Coleoptera chrysomelidae (Gastrophysa
viridula
) on a mint leaf. Length 10 mm.

Figure 9 – Scarlet Lily Beetle (Lilioceris lilii).
It feeds on lily leaves. Length 9 mm.

Figure 10 - Coleoptera curculionidae (weevils)
(genus Phyllobius) on a rose leaf.
Length 8 mm.

 

MINT IS SO GOOD!
Mint is such a perfumed plant that at times it betrays its presence by the intense odour that it emanates and that diffuses in the air. When you find mint observe the leaves well. If they are partly eaten stop and search for the possible presence of a beautiful emerald green beetle: the Gastrophysa viridula (Figure 8) or its larva which is the colour of lead. If you don’t see either of these, with the aid of a pocket mirror, look for the insect or its larva under the leaves because it often hides there. This beetle feeds on different mint species, amongst which Mentha piperita and other wild mint species. This is an amazing animal both to observe and to photograph. Its splendid green shell is decorated with numerous small cavities which give it an even more pleasant aspect. Be careful however, because if you move the plant this insect falls to the ground and is then often difficult to find. It is however a very tranquil insect, it moves slowly and if you want to observe it comfortably, put it in a Petri dish with a mint shoot.

FLIES AND HOVERFLIES
The flies do not allow themselves to be caught easily and the hoverflies are amongst the most capable of all the flying insects. They are in fact able to fly remaining still in space, move suddenly to the side, take off at lightening speed, fly backwards etc. They exploit this ability to chase each other, perhaps for fun. The hoverflies are very beautiful insects to watch. They resemble somewhat the wasps (Batesian mimicry), but have a more squat body, similar to that of a fly. Due to their colour they can be mistaken for wasps, but they are not wasps and do not sting. They can be distinguished from the wasps also because of they have a less slender body and large eyes. The hoverflies also rest on the flowers where they feed on pollen and nectar. With a telescope, you can observe resting flies and hoverflies. If you slowly bring your finger near to a hoverfly, they will sometimes rest on it. Recently, I observed a hoverfly that was resting on a flower. With the anterior legs it cleaned the anthers and with the proboscis it ate the grains collected. The speed with which it fed was astonishing and at the end it was difficult to find any pollen on the flower. Another hoverfly, very similar to the previous one, fed instead on the nectar that it found at the bottom of the calyx of the flowers.

Figure 11 - Hemiptera (Pyrrhocoris apterus).
Length 8 mm without antennae.

Figure 12 - Dragonfly commonly
known as "damselfly".

Figure 13 – Coleoptera on a strawberry flower.
Length 2.5 mm without antennae.


DRAGONFLIES
The dragonflies are probably the best fliers of all the insects, capable also of flying laterally and backwards. They exploit this ability to capture other insects in flight on which they then feed. Dragonflies have the habit of always resting on the same blade of grass or on the same reed. To observe them, approach slowly. You can also observe them using a telescope and you can photograph them with a telescope, but if you are cautious and patient enough, you can also photograph them close up. Cats are also interested in dragonflies and attempt to capture them. In the summer a couple of years ago, one of my cats captured a dragonfly and had damaged it badly before eating it. The head remained, having most likely become detached at the moment of capture. I took it for observation under the microscope and with horror I saw that it was still alive, moving a little according to the movements given to the neck muscles; it still moved the jaws. In its eyes it was still possible to see the “pupil” (the dark spot visible in the centre of the composed eyes is an optic effect due to the directionality of the ommatidia). The faceted eyes of a dragonfly are beautiful to observe under the stereoscopic microscope. Amongst the dragonflies, we must remember the damselflies (Figure 12), characterised by a graceful aspect and a slower flight. When they are at rest they do not hold the wings open like other dragonflies, but keep them together vertically above the body.

The nymphs of the dragonfly live in water. These are equipped with the so-called mask: a pair of jaws at the end of an articulated arm which at rest lies folded under the head of the animal. When a small prey such as an aquatic crustacean passes close enough to the nymph, the mask is suddenly thrust forward, the prey is impaled on the claws and eaten straight away. The speed of the movement of the mask is such that it is almost invisible to the human eye.

INSECTS ON FLOWERS AND LEAVES
With a little bit of caution, it is possible to observe and also photograph insects searching for nectar. You can see many bees and wasps, but also bumblebees and many other similar insects. If you are fortunate, you can also observe Bombylius major, a squat and hairy fly that resembles a bumblebee. This sucks nectar while in flight, without resting on the flower. It has a very long proboscis and passes rapidly from one flower to another. This insect is little bigger than a fly, but there is another more than 20 mm in length that can fly skilfully whilst sucking nectar without resting on the flower. This is a moth, a Lepidopteran from the Sphingidae family, Macroglossum stellatarum, a formidable flier. If you look carefully at the inside of a flower, you will often see small insects a couple of millimetres in length and only a few tenths of a millimetre in width. Some of these have wings folded, others have thin wings formed of small feathers. At the base of trees, under the bark and above all on plants of hibiscus, these Pyrrochoris (Figure 11) are often present in large numbers.

THE PRAYING MANTIS
The elegant bearing of the mantis makes her the queen of the fields (Figure 1), but be careful: her elegance certainly doesn’t make her a kind insect, in fact, the mantis is a carnivore and feeds on other insects that it catches and holds with its forelegs. These legs, which the animal keeps folded but ready to capture prey, are covered with spines. That the mantis is a cruel animal her partner can attest as he is often devoured by the female after mating. In this way, the mantis procures a good reserve of protein for producing eggs. You can find mantids in the grass in fields, on bushes and among the branches of trees where they search for prey. The mantis is often well-camouflaged in its environment and it is difficult to note its presence. Under the stereoscopic microscope, the mantis reveals marvellous compound eyes, a neck which permits the head to rotate and incline as though it were human, but it is certainly not human if you think of the legs equipped with powerful prey capture systems. Observation of this animal with the stereoscopic microscope is particularly interesting, especially when it is devouring prey. You can raise mantids, offering them flies and other insects as food.

PHOTOPHOBIC ANIMALS
If you lift a stone, a piece of wood or other flat objects left on the ground for a period of time, you will witness a general stampede: millipedes, centipedes, beetles, woodlice, mites and insect larvae escape frightened, while earthworms and slugs also do everything possible to hide themselves. These are animals that flee from light and take refuge in damp shaded environments. If you have a garden, find a piece of wood or a tile, place it on the ground and leave it there for a few months. When you want to observe the animals that have taken shelter under it, equip yourself with some Petri dishes to capture them. Unfortunately, you will manage to capture only a few because they escape as soon as they are uncovered. The woodlice (or Oniscidea) (Figure 14), the millipedes and the lithobiomorpha (Figure 15) are certainly interesting to see. The woodlice belong to one of the orders of terrestrial crustaceans (pylum Arthropoda, subphylum Crustacea, class Malacostraca, order Isopoda). At times, even the ants create their nursery under a piece of wood or a flat stone. When you raise the roof of this “crčche” the ants run to take the larvae and place them in shelter within the nest in a humid and dark area.

 

Figure 14 - Woodlouse. Length approximately 6 mm.

Figure 15 - Lithobiomorpha.

Figure 16 – Oak leaf attacked by parasites.

If you take a walk in the woods, collect a little soil. When you return home, put a layer of this soil on top of a mesh sieve with holes a couple of millimetres in size. If you turn on a lamp above the soil, some small animals will pass through the sieve to escape from the light and with a funnel you can make them fall into a glass jar (keep the funnel and jar in the half-light). These animals are usually primitive insects. After you have observed these animals, you could use them to make permanent preparations to observe under the microscope. In order to do this, throw the animals in boiling water where they will die rapidly and stretch out their legs. Dehydrate them by placing them in different, successively more concentrated, alcohol solutions until you arrive at pure alcohol and then colour them with eosin, dehydrate them again and place them in xylene. On every slide, place one or two drops of mounting fluid and an animal still wet with xylene and cover with a coverslip.

PLANT PARASITES AND DISEASES
An important use of the stereoscopic microscope is for the inspection of the diseased parts of a plant to identify what ailment it is suffering from (Figure 16). You can make out small harmful animals such as the red spider mite, capable of causing the leaves of a plant to literally shrivel up, even leading to the death of the plant. In other cases, you can see hemiptera such as the mealybug. Always remember to place the findings on a sheet of paper that you throw in the bin together with the sample after concluding your investigations. Don’t throw out this material in the environment as you could propagate the infection.

With the aid of an appropriate text, you can recognise the principal diseases and any parasites present and you can try to cure the plant. Many of these parasites are small in size, as in the case of the aphids, the mealybugs and the red spider mites. I have already hinted at how interesting it is to observe aphids, and the mealybugs are also beautiful to see and are often covered with white wax filaments. Collect the affected parts of the plant and observe them under the microscope paying attention to not contaminate objects and other plants. Applying any possible treatments and removing the parasites or pruning the infected parts, you can heal the plant and your family will rather esteem you, but don’t delude yourself.

A LESSON ON THE INSECTS
If you are a teacher, in spring, take your students to a flowering field to observe plants and insects. In the absence of a microscope, give the children a magnifying lens so that they can better see the samples that they find. Children enjoy observing nature very much and you can profit from their curiosity to describe the principal characteristics of the animals and plants that they encounter.

In class, you can describe the differences between the structure of the insects and that of human beings: external/internal skeleton, six legs/two legs and two arms, wings/no wings, compound eyes/”simple” eyes, antennae/nose etc. This is also the occasion for speaking of the role of insects in nature and their usefulness to humans and other insects as pollinators, producers of honey, in the fight against insects which are harmful to crops and as shredders and decomposers of organic refuse.

DON’T STRESS THE ANIMALS!
 Walking in a field, you will see many insects on the flowers. But, if you approach to observe them more closely, many of them will fly away. This problem increases if you move very close to the animal to observe it with a magnifying lens. If you capture an insect and put it in a container to observe it at home, the animal will most likely become extremely agitated. Under these conditions, you will find it very difficult to observe it. It is also true that some insects will allow you to come close to them and you can also take them in your hand without frightening them, permitting you to observe and photograph them calmly, but for other insects it is necessary to find a different solution.

At home you have the advantage of having a microscope and other equipment at your disposal that you cannot easily take with you into a field. You can also carry out dissections and take photographs of your specimens, having all of your equipment close at hand and better and adjustable light conditions. The biggest inconvenience of bringing live animals home is that they usually arrive frightened and with a great desire to escape. Furthermore, removing an insect from its environment is not always the best way to examine and photograph it. As if this wasn’t enough, the air inside the house is very dry, particularly when lamps are used for illumination and so the insect can die of dehydration.

Rather than bringing the animal home, you could take the microscope to the field together with a small table and a folding chair. Place these objects under a tree so that the bright sunlight does not disturb your observations. Under these conditions, you can observe still fresh flowers and animals which have been only recently collected and so are little affected by captivity. You can even take with you photographic equipment or a kit to dissect and prepare the samples. You can collect insects that tend to escape in a Petri dish, for example wasps or ants. You could try to calm them offering them a drop of honey mixed with water.

Continuing with the idea of reducing the disturbance to the animals under examination, you could bring the microscope close to the animal, leaving the insect where it is resting and not touching it. In order to do this, you will have to remove the base of the microscope and substitute it with a tube that is sufficiently long to be planted into the ground and reach the microscope that is positioned to observe the sample. Often the plants and flowers sway as a result of the wind making observation difficult. To minimise this problem, you could take a rod with you to insert into the ground so that the plant remains still. If necessary use a clothes peg. In many cases this system works. For example, aphids do not become frightened, but a butterfly will become very alarmed at the sight of all this movement and will certainly fly away.

To observe the more timid animals or those which are simply long-sighted, as I have already suggested you can use a telescope. To see an object at a distance of a few metres with a telescope, it is necessary to equip the instrument with an additional tube in order to increase the distance between the objective and the eyepiece. The length of this tube has to be determined experimentally. Another method is to put an additional lens in front of the objective of the telescope. This system works very well and will allow you to carry out observations and to take photographs of small animals comfortably and without disturbing them in any way. Not only, but you will also leave the animals in their natural habitat and even the photographs will benefit. For further information, see the article "Equipment for Explorations” of this guide, in the paragraph on telescopes.


INFORMATION ON THE INSECTS

THE BODY
The insects derive from ancestors similar to the present day Annelids and are also characterised by the subdivision of the body into segments. Differently to the annelids, the segments of the insect body differ greatly from one another. The body is divided into three principal parts: head, thorax and abdomen. The head is formed of 7 segments and is specialised in sensory activities and in the intake of food. On the head there are normally a pair of compound eyes and a pair of antennae that are sensitive to odours. There are also the mouthparts that in every insect are specialised for the food that it feeds on (for example: the piercing and sucking apparatus of the mosquitoes, that of the plant bugs, the proboscis of the butterflies, the short proboscis of the flies). The elements that compose the compound eyes are called ommatidia, and each one is formed of the cornea, crystalline lens, pigmented cells and the retinula. The eyes of an insect do not give detailed vision, but are highly adapted to detect rapid movements.

The thorax is composed of 3 segments, each of which is equipped with a pair of legs. The final two segments can also each have a pair of wings. The abdomen is formed of 6-12 segments, lacks legs and contains the reproductive organs. On the abdomen of grubs there are pseudolegs that are then lost during metamorphosis. Normally, insect species are very different from one another and their body organs are highly specialised and vary widely in form and function. For example, the large eyes of the dragonflies are specialised in following the rapid movements of the prey that they capture in flight and in some Ichneumonidae the ovipositor is capable of drilling through wood.

The coelom of the Arthropods and therefore also that of the insects is reduced and consists of only the cavity of the excretory and reproductive organs, called the haemocele. The heart has a tubular form and pumps the blood (haemolymph) in the haemocele that surrounds the internal organs. In the insects, the haemolymph does not flow in vessels but flows freely in the body, with the exception of the tract composed of the heart. The blood of insects is rarely red, in fact, due to their small size, the tissues of the animal are well oxygenated by diffusion across the tracheae and so they do not need haemoglobin. The larvae of the chironomids are an exception as they live in water which is often poor in oxygen.

The nervous system of the insects is decentralised and formed of a “brain” in the head, a nervous centre beneath the digestive tract and a chain of ganglia distributed along the abdomen.

THE LEGS
The legs are composed of 5 principal parts: the coxa or hip (attached to the body), the trochanter (inconspicuous), the femur (the most robust), the tibia (long and thin), the tarsus (normally formed of 5 segments) and finally the pretarsus (formed of some of the following parts: two claws, an arolium or an empodium, two pulvilli). The legs, as with other organs of the insect, are specialised for certain types of movement. They can be adapted for swimming (diving beetles), skating on the water surface (pond skaters), running (ants), jumping (grashoppers), climbing (ants, flies), grasping (mantids) and digging (mole crickets). Often, the legs of insects are also adapted for cleaning the eyes and the antennae. In the bees, the hindlegs are equipped with curved hairs that form a “basket” for collecting pollen.

The tarsus of the legs is particularly interesting. Its last segment possesses various grasping organs. The most diffuse of all is formed from a pair of claws. Between these claws there can be a pad called the arolium. Some diptera have two additional pads positioned laterally with respect to the arolium called pulvilli. In many flies, in place of the arolium there is a central bristle called the empodium. In the penultimate tarsal segment of many insects, there are other pads called plantulae. When the claws don’t manage to grasp, they are immediately assisted by the different pads. All of these organs allow the insects to walk indifferently on the rough or smooth surfaces of horizontal and vertical walls and on ceilings.

In many species, the adherence of the arolium to a smooth surface is facilitated by a thin film of liquid. In other species, the arolium and the pulvilli are formed of minute fibres that adhere to surfaces by means of van der Waals forces. In this case, they resemble brushes. Grasping structures such as these are also present in many wall spiders and geckos. The adherence capacity of this mechanism is extremely elevated and capable of supporting up to several hundred times the weight of the animal. A twist of the tarsus at a certain angle is sufficient for the leg to lose its grip. In the bigger animals, the bristles are finer and more densely arranged.

 

 

Figure 18 – Extremity of the tarsus of Pyrrhocoris
seen from above. Field = 0.84 mm.

Figure 19 - Extremity of the tarsus of Pyrrhocoris
seen from the side. Field = 0.84 mm.

In Figures 18 and 19, the last segment of the right foreleg of the Hemiptera Pyrrhocoris is visible (like that in Figure 11). Note the claws and pulvilli. The central arolium is missing, but in its place in Figure 19 you can see what should be an empodium formed of two bristles.

To understand how an insect walks, it is necessary to keep in mind that the minimum number of legs needed to keep a table stable is three. The insects have double the number of legs. The reason for this lies in the fact that when they walk normally the insects keep three legs still while they move the other three forward.

Capture flies and other insects to examine the grasping organs of their legs. To do this, you can put the insect in a Petri dish. Turning it upside-down, you can observe the insect on the ventral surface and easily see the legs. Even the humble drosophila, the common vinegar-fly, has a tarsus equipped with a pair of claws and two pulvilli. Compare the legs of different insects and describe what you see.

THE WINGS
As I have said, the insects are the only invertebrates capable of flight, although not all insects can fly. The wings are also one of the principal characteristics for the classification of these animals and the names of the main orders describe in some way the type of wing possessed. For example, the term Diptera indicates: “insect with two wings” (flies, mosquitoes), Lepidoptera: “insect with scaly wings” (butterflies), Hymenoptera: “insect with membranous wings” (ants, bees, wasps), Coleoptera: “insect with cased wings” (ladybirds etc.), Hemiptera: “insect whose forewings are halved” (bugs, cicadas, scale insects, aphids). In general, the insects have 4 wings borne on the second and third thoracic segments.

The different insect orders and species display different flying abilities. The dragonflies and some flies and hoverflies are capable of flying whilst remaining motionless in the air, of suddenly moving forwards or sideways, of turning brusquely and even of flying backwards. The flies are able to land on the ceiling. There are also insects that suck nectar from flowers without even landing on them. The Coleoptera, instead, fly badly and before taking flight they need lengthy preparations, often climbing up to the tip of a branch. The bees and wasps fly well, as do some butterflies. The flying ability of the butterflies can be appreciated when they chase each other.

In the Coleoptera, the first pair of wings (the eleytra), are transformed into a protection for the second pair which are used to fly and which, when at rest, are folded under the eleytra. The wings of the butterflies are instead covered with minute coloured scales. The wings of very small insects such as the Thysanoptera (thrips) are endowed with soft hairy fringes. In the Diptera, the second pair of wings is transformed into a couple of halteres or balancers. You can easily see these in the flies and the mosquitoes. In the Orthoptera the forewings, called the tegmina, are thickened and serve to protect the hindwings. The grashoppers emit a call by rubbing the tegmina against the femurs of the hindlegs. To emit its call, the crickets instead rub the tegmina against each other. One of these is called the file and the other the scraper. In general, it is the males that produce these calls.

REPRODUCTION AND DEVELOPMENT
The insects are in large part diploid and their gametes are haploid. The eggs of the insects have a surprisingly wide variety of forms and colours. It is enough to think of the eggs of Chrysoperla carnea, placed at the apex of a long stalk. Often, the eggs are deposited in compact batches (e.g. mosquitoes and bugs). In other cases, they are laid inside nests made of leaves or inside oothecae (e.g. mantids). The eggs of the icneumonids are usually deposited in the bodies of larvae, grubs or spiders. Normally, the eggs are left to themselves, with the exception of the social insects (bees, wasps, ants, termites) which instead feed and care for the larvae until they reach the adult stage.

Depending on the type of metamorphosis that they undergo, the insects are divided into ametabolous, heterometabolous or holometabolous.
The ametabolous insects do not undergo a true metamorphosis, and are generally primitive insects.
The heterometabolous insects are those whose juvenile forms are very similar to that of the adult. The young usually lack wings. In the first stage of their development they are called nymphs. This group includes the grasshoppers, mantids, roaches, bugs etc.
The holometabolous insects undergo a complete metamorphosis and this is a characteristic of the more highly evolved insects. These insects go through 4 stages: the egg, the larva, the pupa (or nymph) and the adult. The terms grub and chrysalis are used for the larva and the pupa respectively of the butterfly. Usually, the larvae live much longer than the adults, but are often exposed to the attack of predators. For this reason, they do everything possible to pass unobserved, as for example the larvae of the stoneflies that construct a case of vegetation fragments and stones. The pupa is immobile and is subject to great transformations (for example the chrysalis of the butterfly). Often, the adult eats little or nothing and when it does, it normally consumes nectar or pollen, as in the case of the butterflies. The dragonflies instead are carnivorous both in the larval stage and as adults.

Amongst the aphids, there are individuals destined to increase the population (through parthenogenesis, asexual reproduction where non-fertilised eggs develop) and others destined to exchange genetic material (through sexual reproduction). Many species also live on different plants throughout the year. From eggs laid in the autumn, the females are born that reproduce actively by parthenogenesis (with the microscope, it is possible to see the spectacle of the young while they are being born). These females also actively reproduce asexually. These also produce winged individuals which have the task of propagating the species. At the end of summer, males and females appear that reproduce sexually. Eggs are laid that overwinter and from which new colonies are born.

As previously stated, from a colony some winged individuals can migrate to another plant upon which they continue to reproduce asexually. At the end of summer, some of these individuals can return to the plant of origin where the normal males and females mate and produce eggs which will overwinter.

The observation of an aphid colony is a true spectacle. You can observe the adults, other individuals that have just been born, females giving birth, winged individuals, ants intent on stimulating the aphids to produce the honeydew on which they then feed, ladybirds, green lacewings and their respective larvae as they eat the aphids, aphids infected with parasites and others that have been immobilised, aphids during the moult, the exuviae etc.

 

Figure 20 – Complete development of the silk caterpillar (Bombix mori):
egg, various larval stages, exuviae, chrysalis with cocoon, adult male
 and female, mulberry leaf. Seen from below. (Photo Sini).

Figure 21 – Cryptic camouflage of the butterfly Kallima inachus, China. The “K” indicates
an insect with closed wings resting on a branch. The underside of the wings resembles a
leaf and its veins. The spur of the wings resembles the petiole of a leaf. (Photo Sini).

BEHAVIOUR
The observation of the insects should regard not only their form and colour, but also their behaviour. The behaviour of the insects responds to certain fundamental stimuli such as the search for food, defence from predators (camouflage, disguise etc.) and reproduction (chemical, visual and acoustic signals). The social insects provide evidence of very complex and coordinated behaviour that regards nest construction, care of the larvae and queen, in certain species the cultivation of mushrooms, etc. The study of insect behaviour is very different to the microscope observations and constitutes an autonomous and complementary field of investigation.

CLASSIFICATION
For various motives, the classification of living organisms often differs from one author to the next and even the university texts do not agree completely. For this reason, it is useful that whoever uses a text that reports a classification reads attentively how that classification was conceived, in order to use it conveniently without remaining disorientated and without feeling obliged to take the part of one specialist or other.

In this article I have referred to a classification reported in the atlas-text of Chinery [201] because this is a very diffuse manual amongst entomology enthusiasts. With regard to this classification, it is necessary to keep in mind that in relation to the animals the term "Division" has a taxonomic value which is inferior compared to that of the plants. In fact, for the plants the Division is the same as the Phylum level for the animals.

 

CLASS

SUBCLASS

DIVISION

ORDER

EXAMPLES

INSECTA

Apterygota

 

Thysanura Bristletails
Diplura Two prolonged bristletails
Protura Proturans
Collembola Springtails

Pterygota

Exopterygota
(or Hemimetabola)

Ephemeroptera Mayflies
Odonata Dragonflies
Plecoptera Stoneflies
Grylloblattodea Rock crawlers
Orthoptera Grasshoppers, Locusts, Crickets
Phasmida Stick insects, Leaf insects
Dermaptera Earwigs
Embioptera Web-spinners
Dictyoptera Cockroaches and mantis
Isoptera Termites
Zoraptera Zorotypus
Psocoptera Psocids or booklice
Mallophaga Bird lice
Anoplura Lice
Hemiptera Bugs, Cicadas, aphids, scale insects
Thysanoptera Thrips

Endopterygota
(or Holometabola)

Neuroptera Alder flies, lacewings, snake flies
Mecoptera Scorpion flies
Lepidoptera Butterflies and moths
Trichoptera Caddis flies
Diptera Flies, mosquitoes
Siphonaptera Fleas
Hymenoptera Ants, bees, wasps
Coleoptera Beetles
Strepsiptera Stylopids

 

The insects are hexapod animals, equipped with an exoskeleton and a pair of antennae. The first great subdivision of the insects is that between the Apterygota (rather primitive insects without wings) and the Pterygota (more highly evolved insects which either have wings or had wings that they have lost in the course of their evolution). The Pterygota are in turn divided into Exopterygota (insects with wings that develop on the outside of the body) and Endopterygota (insects with wings that develop internally in the larva). They possess an exoskeleton which the Arthropods must periodically abandon (exuvia), as it becomes too tight and impedes their growth. The underlying skin is more elastic and deformable and the animal profits from this to increase its size before this also becomes rigid. This process is called the moult. It is possible to find the exuviae of many insects in the grass and also those of spiders. It is worth while collecting and examining them.


IN-DEPTH

While you carry out your explorations with the microscope, it is always with the goal of strengthening your capacity to better understand that which you see, and to become more autonomous in your research you should study the following subjects:

- the origin of the Earth and the characteristics of the geological eras;
- the conquest of dry land by plants and animals;
- the evolution of the Invertebrates and the appearance of the coelom;
- the principal characteristics of the different Invertebrate Phyla;
- the classification of the Arthropods and in particolar of the Insects;
- the principal characteristics of the different Insect orders.

During this study, it is useful to compare how the same functions are carried out in the Invertebrates and in the Vertebrates.

You can study some of these topics in a high school textbook, as for example the one indicated in [001]. For an introductory study of the Insect world, get a specific textbook which describes also their behaviour. The text [201] will instead serves as a guide and atlas for recognising the insects. Furthermore, it will serve to know the characteristics of the species that you encounter. A zoological atlas of the invertebrates is useful for the numerous illustrations of the anatomy of these organisms and for the description of their evolution and differentiation. The text [004] and [005] can be useful for breeding little animals, for their anatomy and other experiments. In [0007] you will find also beautiful drawings on insects.


CONCLUSION

The Arthropods represent an alternative evolutionary line to that of the Vertebrates. The solutions that they have adopted to face their problems are often fantastic and even inspire the most modern human technologies. Even the field of the small terrestrial animals has shown to be incredibly vast, offering to our stereoscopic microscope innumerable arthropod species, each of which is often of great interest from the aesthetic and cognitive point of view and for the form, the decoration, for the often sparkling colours with metallic reflections and for the behaviour and anatomical particulars highly adapted to their function. In this regard, we saw the example of the tarsus of the legs, but wings, eyes, antennae and mouthparts are all extremely diverse organs in the various species and interesting to examine and compare. For every insect that you observe, you can compile a file with photographs and drawings in which you note the description of the animal, its form and behaviour. Also in this case, you have at your disposal an infinite field of subjects to observe under the microscope.


BIBLIOGRAPHY

 

201

Chinery M.; Insects of Britain and Northern Europe; Collins Field Guide); 320 p.

This is an atlas of European insects and includes 800 illustrations and 350 drawings. It also supplies information on the structure of the insects, their reproduction and their development, and methods of collecting and conserving insects. It details the classification of these animals and describes the characteristics of the different orders. The illustrated tables of this volume are very useful for recognising the more common species.

202

 


Look also at Internet works and references of general character which are indicated in the presentation article of this guide.


INTERNET RESOURCES

2001 - http://en.wikipedia.org/wiki/Insect  Wikipedia, Insects.
2002 - http://www.britannica.com/EBchecked/topic/289001/insect Arthropod class (class Insecta or Hexapoda) from the Encyclopćdia Britannica
2003 - http://www.uky.edu/Ag/Entomology/ythfacts/entyouth.htm  Entomology for Kids and Teachers
2004 - http://www.uky.edu/Ag/CritterFiles/casefile/4Hent/oldunit2.pdf  Lessons and report cards on insects
2005 - http://bugscope.beckman.uiuc.edu Bugscope, insects under the scanning electron microscope (SEM)
2006 - http://parasitology.informatik.uni-wuerzburg.de/login/n/h/j_436-98-5-2006-01-14-100.html.linked  Legs of Insects under the SEM
2007 - http://www.kendall-bioresearch.co.uk/bugindx.htm  Insects and other arthropods
2008 - http://www.entomology.cornell.edu/Faculty_Staff/Danforth/322LabManual/lab05.pdf  Arthropod appendages and locomotion
2009 - http://www2.entomology.cornell.edu/Faculty_Staff/Danforth/322LabManual/322Lab.html  Eickwort's Manual of Insect Morphology

Internet Searches: arthropods, insects, (various orders, families, genus and species), myriapods, arachnids, entomology, insect structure, arolium insects, pulvilli).

 


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