Natural History Illustration: Anatomy of an insect January 2015

Story posted: Thursday, 15. January 2015 by Lizzie Harper

Insects are my favourite creatures, and I love illustrating them in my natural science commissions.  It always helps to fit form to function, so here's a brief overview of the parts of any insect which should help anyone about to do an entomological illustration.

Insects are invertebrates; they sport an external skeleton rather than bones on the inside.  Their limbs are jointed, they're cold-blooded, they have six legs and (mostly) two pairs of wings, and a body split into three sections; namely the head, thorax and abdomen.

Lizzie Harper natural history illustrator anatomy of an insect coleoptera beetle

Basically, any insect can be split into three parts.  A head (generally box-shaped and containing the insect's sense organs and mouth), a thorax (the middle, where the wings and legs attach), and the abdomen (the rear section where breathing and digestion/ excretion/ reproduction occur).

The Head

Insects sense the world around them with eyes, and these can be simple or made of many lenses (compound).  Some insect eyes merely detect light and dark; others can see all the colours we see, plus ultraviolet light.  A top insect predator, such as a dragonfly, has thousands of lenses building up its compound eye and its thought their eyesight is pretty acute.

Antennae are attached to an insects head and let it smell, taste, and touch the world around it.  They pick up on vibrations too.  Like most insect features they vary wildly from animal to animal; consider the incredibly long antennae of a blind cave cricket next to the furry ones of a moth, picking up pheromones that let it locate a mate.

Insect's mouths are there to allow the insect to feed.  However, some insects don't eat as adults (the mayfly for example), and have no mouths at all.  With food stuff as varied as leaf-litter to other insects, seeds to wood and meat, insect mouths have evolved into a regular toolbox for feeding.  Crushing, slicing, sucking through straws, chewing through trees - insects have evolved them all.  Mouths also can be used to get rid of waste products, deliver bites, and produce silk and venom.

The Thorax

This is the power-house of the insect.  Wings and legs are attached to this box-like structure; and within lie the muscles to drive locomotion.

Legs are segmented and are are used for walking, jumping (think of the flea), swimming (a diving beetle comes to mind), and digging (take a look at a mole cricket).

Lizzie Harper natural history illustration of Acrocinus longimanus beetle

This Harlequin beetle, Acrocinus longimanus, has extraordinary legs which it uses to attract females, and to get across tree branches in its South American rainforest home.

Feet are good for gripping and for cleaning the insect; they also may have sensitive hairs on which help the insect feel danger from behind.

Insects have two pairs of wings; forewings (at the front) and hindwings (behind).  These are used for flight and display.  Some insects have lost their wings entirely (like the ants) whilst others have adapted them enormously.  Beetles have evolved their forewings into tough coverings, or elytra, which enable them to exploit all sorts of habitas without compromising their ability to fly.  Flies meanwhile have adapted their hindwings into tiny lolly-pop like structures called halteres which work like gyroscopes, allowing complete control over rotation and direction during flight.

Lizzie Harper natural history illustration blue bottle fly with haltere

The blue-bottle fly, Calliphora vomitoria, showing halteres.  These are easiest to see on a crane fly (daddy long-legs).

The Abdomen

Insect abdomens are the site of digestion, excretion, circulation, and respiration.  Their hearts are within their abdomens, and although different to ours they do the same job; pumping blood around the insect's body.  Insects breathe through their abdomens too, through small holes called spiracles which connect to internal tubes (trachea). These in turn connect to smaller tubes (thracheoles) which terminate in waater filled spaces of 1micrometer or less. This is where gas exchange can occur.

Lizzie Harper natural history illustration lepidoptera moth tobacco hornworm showing spiracles

Caterpillars are good subjects if you want to see spiracles; they often appear as dark spots along the animal's side.  This one's a tobacco hornworm (Manduca sexta).

The most wonderful thing about insects is their adaptability, they've taken this basic body plan and run with it in thousands of different evolutionary directions.  To see if you've got the basics sorted, try to identify all the key insect features on the insect illustrations below.

Lizzie Harper natural history illustration insect pen and ink images

There's so much to say about insects, and how magnificent they are, that it'll doubtles be a topic I return to.

On another topic entirely, I'm afraid I've had to turn off the "comments" button on my blogs due to spamming.  If you would like to give any comments or feedback, please do so on my Facebook page or Twitter account or email me at  Many many thanks; and apologies.

Category:  Zoology Terms and Anatomy   |   Comments:  0   |   Viewed:  5184

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