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Dr. Miller ENY3005/5006 Principles of Entomology, University of Florida Includes material from the intro. entomology course taught by Dr. Douglas Emlen and from The Insects, Authors: Gullan & Cranston.

WEEK 2: EXTERNAL MORPHOLOGY NOTES

The insect exoskeleton is a non-cellular, non-living covering called the cuticle which is secreted by the outer cell layer of the body – the epidermis.

The insect exoskeleton, like a medieval suit of armor, is composed of rigid, protective plates called sclerites, which are joined by softer regions of membranous cuticle.

There are some major consequences of having one’s skeleton on the outside, and these are detailed in your lecture slides. Regarding #5, exoskeletons place an upper limit on insect size. As the volume of an animal increases, the thickness of the support structure that is required increases disproportionately quickly – the exoskeletons get prohibitively thick and heavy fast! Some of the largest, heaviest beetles in the tropics can only fly at night and in the rain because they can get overheated when flying. It takes a lot of effort to get off the ground! Marine environments, however, allow for invertebrates to grow larger. The water pressure helps support the animal, thus, the exoskeleton does not need to be as thick (think lobsters and giant crabs).

The Integument

The integument has three layers: a living cell layer, the epidermis, a non-cellular basement membrane, and the cuticle, which lies outside the epidermis. The epidermis is only one cell-layer thick. It is extremely important during molting and metamorphosis. This layer secretes the cuticle and the compounds which harden, or SCLEROTIZE, the cuticle.

Interspersed among epidermal cells are dermal glands which secrete defensive compounds, pheromones, silk, etc. that are emitted to the outside world.

The Cuticle

The cuticle protects insects. It often comprises up to one half of the total dry weight of insects. The cuticle can be divided into two primary layers, the epicuticle and the procuticle. The procutical can itself be divided into two layers, the exocuticle and the endocuticle.

• The epicuticle is thin, but structurally complex, and very important. The inner layer of epicuticle is protective, made of cross-linked lipoproteins, and is impermeable to many chemicals. It is penetrated by wax canals. Outside of this layer is the wax layer of the epicuticle which is made from lipids and hydrocarbons – this layer waterproofs the exoskeleton. Outside of the wax layer is the cement layer, which is secreted by dermal glands. This outmost layer determines the surface properties of the cuticle and protects the more delicate wax layer beneath it. The cement layer is not thick enough to provide strength to the cuticle… you can think of it as the shell on an M&M – protective, but not strong.

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you see shiny, metallic colors, these are generally structural colors. 2) Permeability. Terrestrial insects are continually faced with the problem of losing water. Their

small body sizes render them particularly susceptible because they have surface areas that are very large relative to their small body volume. One important role of the cuticle is to keep water inside the animals.

a. The wax layer of the epicuticle is impermeable to water, all of the millions of pore canals do not extend through this layer. Dermal gland ducts do pass through, but these can be closed.

b. If you dissolve part of the wax layer with a solvent, the insect will dehydrate and die. Diatomaceous earth, a common “green” insect killer, works by stabbing insects with tiny blades (made of diatoms), causing them to dehydrate.

c. Some insects do not encounter water stress (they live in very moist environments), and many do not have a wax layer. And, sometimes a little water loss isn’t bad – desert cicadas use “sweat” pores to cool down, but they get ample water from their diet of plant juices (bad for the plants!).

3) Barrier to pathogens or parasites. The cuticle is quite effective at preventing entry of most bacteria, viruses, and protozoans. However, the cuticle is not effective against some fungi, nematodes, and the ovipositors of parasitoids. These are all good at locating soft spots (non-sclerotized membranes between sclerites) and getting in that way. Some predators and parasitoids find the mouth, anus, or spiracles to get inside… and this is an important area for current research into possible biocontrol agents.

4) External integumentary processes. Insects display an impressive array of cuticular extensions. There are four basic types, and all are made from sclerotized cuticle.

a. Spines: multicellular and made from undifferentiated epidermal cells.

b. Setae (also called hairs or trichoid sensillae): also multicellular, but they involve highly specialized cells, the nerve, basal, tormogen, and trichogen cells.

c. Acanthae: unicellular. A separate cuticular extension is secreted by each epidermal cell.

d. Microtrichia: similar to acanthae, but so small that many extend from each cell.

Setae sense much of the insect’s tactile environment. The other three types are fixed, immobile, and have little or no sensory input. But, they still serve key functions. For instance, dung beetles use spines to dig and to brace themselves inside tunnels during fights. Acanthae and microtricia can be used to produce sound, e.g. the stridulatory song of crickets. They rub ridged cuticles together to make the song. Spines can also be used for protection, because they make insects hard to swallow.

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WEEK 2: EXTERNAL MORPHOLOGY STUDY GUIDE

Study Questions and Objectives

What are five consequences of an exoskeleton (versus endoskeleton)?

Carefully explain the major components of the cuticle (listed in Study terms below) and their function.

Discuss five physical properties of the cuticle.

What is a head used for (4 functions)?

Name and sketch your favorite three types of antennae.

Explain how male mosquitoes detect females and which organ (on which part of the antennae) is used. Are these males able to detect the wing beat frequencies of young females or other males?

Be able to identify a pronotum and describe some elaborations of this structure

From Emlen reading, know the answers to Assignment #1

Study terms (know what they are and what they do)

Epicuticle, exocuticle, procuticle, endocuticle, basement layer, dermal gland, epidermis

Formation zone, wax layer, superficial layer, outer epicuticle, inner epicuticle, wax filament, pore canal.

Chitin, integument, exoskeleton, cuticle

Setae, spines, acanthae, microtrichia

Sclerotization, Resilin

Elytra, tegmina, hemelytra, halteres

Cerci, ovipositor

Antenna, scape, pedicel, flagellum

Ocelli, stemmata,

Johnston’s organ

Genitalia , Aedeagus

WEEK 2: EXTERNAL MORPHOLOGY NOTESentnemdept.ufl.edu/miller/eny3005/secure/3_notes.pdfA new, larg...

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