Ento. 101 – External Structure I: Intro. and Integument

The next few posts will deal with general insect anatomy. Understanding the body plan of insects and knowing the names of the various parts are essential for identifying, understanding and communicating about them.

This section of Ento. 101 will sometimes refer to the online course, General Entomology, as  presented by Dr John Meyer at NC State University.

I think that many will approach this section with some trepidation, or they may skip it all together. I place myself in the former category – doesn’t anatomy involve volumes of information and memorization of masses of obscure terminology? Isn’t it like learning a completely new language? Yes, yes and almost!

Hoverfly on windflowerTo put myself in the right frame of mind to approach this, I asked myself, “Why do I like bugs?” The answer: “I like bugs because of their variety and complexity, which I can only really appreciate by looking at them closely.” Once I begin this (perhaps with help from a magnifying glass, a macro lens or a microscope), I am filled with wonder. The earwig loses nothing to the elephant in terms of complexity. I could stop there, satisfied with just the wonder. But more likely I will ask more questions. Why is it this colour or shape? What is the purpose? How or why did it originate? Why is it different from other related species? In order to learn more, to communicate with others, to understand the observations and research that has already taken place, I must know the proper terminology. It is not quite as daunting as it first seems – many terms are already in our vocabulary in some form or another, and many complex words are merely trains of connected simple terms. For instance, tongue-tripping words like mesinfraepisternum derives from meso+infra+epi+sternum…

Having said all this…no worries! Even entomologists occasionally need to refer to an entomological dictionaries!

Common terms for Orientation

Some terms that are repeated throughout the descriptions of insect anatomy:

  • anterior – toward the front
  • apical – the tip
  • basal – the bottom or base
  • caudad – toward the tail
  • distal – toward the end
  • dorsal – the top or back-side
  • endo – inside, innermost
  • epi – outside, outermost
  • infra – below
  • lateral – the sides
  • medial – central
  • meso – middle or between
  • meson – an imaginary central line, dividing the insect into right and left sides
  • mesal – toward the meson
  • meta – last
  • posterior – toward the back
  • proximal – toward the base
  • pro – first
  • ventral – the bottom or lower-side
  • ventrad – toward the ventral side

Click to enlarge Notes on my images: I used a conehead katydid as representative of general insect anatomy. Different orders, and families within orders can vary greatly and these variations will be covered in the future sections on diversity. For those interested: I found this katydid in some imported vegetables many years ago, already dead. At the time I popped her into isopropyl alcohol for future reference – which she is now providing. The original colour was green, which has faded completely in the alcohol. I have boosted contrast and saturation in the photographs to make the structure clearer. (Needless to say, she was not hurt in her contributions to this Ento. 101 section… ;))

General Note: I have found that links within a text are both a boon and a bane. It is great to have a web of knowledge instantly at hand, but constantly clicking on links when reading through an article has the potential of side tracking or even derailing the learning process. (Which is the main reason I have been so slow producing Ento. 101 posts!) My humble advice is to read through this article first, and then go back and explore the links later.

General Structure

First, view the slide show by Dr John Meyer at NC State University (NCSU).

Insects are the most successful class of all invertebrates. Like other arthropods, they are defined by the exoskeleton, which both contributes to insect success and limits size. Adult insect bodies have three basic divisions or tagmata – the head, with sensory antenna, compound eyes, ocelli and mouthparts; the thorax, which has the wings and three pairs of  jointed legs; and the abdomen, with spiracles and genitalia. Each tagmata is formed of segments, which can have specialized functions and can fuse together in different ways. Segmentation will be discussed in greater depth in the future section on evolution and development.

The Exoskeleton or Integument (NCSU)

The pronotum, a large sclerite that protects the neck and thorax

The integument (“covering”) supports the insect from the outside rather than the inside, unlike the endoskeleton of vertebrates. The ‘exoskeleton’ has attachment points for muscles and is sclerotized (hardened) in areas to form plates that can protect the internal organs.The plates, called sclerites, are connected by a flexible conjunctivae which allows for the movement of joints and segments.  Sclerites vary in name according to the location. For instance, sclerites on the dorsal side are called tergites, sclerites in the lateral region of the body are called pleurites, and ventral sclerites are called sternites.

There are four features related to sclerites that are important to know about. Apodemes, or internal folds (visible as a suture) and apophyses, internal finger-like projections (visible as pits), provide attachments for muscles and strengthen the integument. Sulci indicate an inflexion (inward-bending or fold) of the cuticle; and finally, sutures, which show at the union of two different sclerites. ‘Lines of weakness’ are cleavage lines for ecdysis, and are sometimes also identified as sutures.

Images by by T. Nathan Mundhenk

Ecdysis in a Cicada

The integument lines internal organs such as the trachea and the fore and hind gut. Externally, it can have setae, spines and scales which are projections produced by the epidermal cells. In different forms, setae can act as tactile receptors, provide camouflage or adhesion and give protection. The integument also protects by retaining and/or resisting water, excluding pathogens and by possessing the body coloration – colors that can show toxicity or distastefulness that predators learn to avoid.

The insect exoskeleton is not just a protective shell – it is a living organ that provides for sensory organs, wings, locomotion, heat exchange and the exchange of gas and other molecules with the outside world (more on this will be in the section on internal anatomy). It places limitations on insects in that it has a limited ability to expand and therefore must be shed in order for growth to occur. The process of ecdysis (molting) of the old integument leaves insects in a vulnerable position until the new integument hardens. (More on ecdysis will be found in the section on life-cycles)

As usual, comments, critiques and corrections are encouraged.

Next, the External Structure II: The Head – in which I do a dissection.

References

Gordh G. and D.H. Headrick.  A Dictionary of Entomology. Cabi 2001.

Romoser, William S. The Science of Entomology, pp. 13-28. Collier-MacMillan 1973.

Wallace, Robert L. et al. Beck and Braithwaite’s Invertebrate Zoology, 4th Ed., pp. 247-250. MacMillan 1989.

Resh, Vincent H. and R. T. Cardé, Eds. Encyclopedia of Insects, pp. 12-15. Elsevier 2003.

This entry was posted in Diptera, Entomology 101, Orthoptera, Syrphidae.

One Comment

  1. Kurt 24 January, 2011 at 9:55 AM #

    Great post Adrian. Exactly what I need 🙂 Thanks.

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