Basic Insect Anatomy

The foundation of entomology is the anatomy of an insect. Arthropods in general have evolved strategies to live successfully under a variety of conditions, and a strong understanding of external and internal anatomy of the insects involved in decomposition will give an investigator a place to start.

The overall body form of an insect is basically cylindrical and elongate and as noted previously, is broken up into three major body regions: the head, the thorax, and the abdomen. The head is made of sclerotized, or hardened, segments fused to form a capsule. It is rigid to protect the brain and give strength to the mouthparts, and houses the sensory center of the organism. The sensory center consists of visual input, through the simple and compound eyes, and olfactory input, through the antennae (Gullan & Cranston, 2009).

Visual input allows the insect to see movement, potential oviposition site, potential food, and find mates. Most insects have two types of eyes: the simple eyes and the compound eyes. The compound eyes, when present, take up the bulk of the insect head, and consist of many individual units or lenses working together to form a single image. This image is supported by individual lenses, called the simple eyes, which give the insect information about light and dark. Working together, the compound and simple eyes allow flies and other insects find carrion (Nation, 2011).

The insect mouthparts come in a variety of forms. Some insects have chewing type mouthparts, which allow that insect to feed on a variety of substrates and chew that food. Other insects have piercing and sucking mouthparts, which allow the insect to pierce through the skin of an animal or the surface of a plant and feed on the internal fluids. Still others have sponging mouthparts, which enable the insect to produce digestive juices into the environment and soak up the resulting digested substance (Nation, 2011). Some insects have siphoning mouthparts which act like a straw and allow the insect to feed on freely available liquids like water and nectar. Some immature insects have specialized mouth hooks in place of the mouth parts, which can scrape food in the environment for eating. The types of mouthparts found in forensically important insects varies based on the type of tissue each insect is feeding upon. Those insects that feed during the early stages of decomposition tend to have sponging mouthparts as adults and mouth hooks as larvae. Those that feed during the later stages of decomposition, when tissue tends to be tougher, tend to have chewing mouthparts (Mullen & Durden, 2002).

The thoracic region of the adult insect has three pairs of legs, and often has wings. The legs are five-segmented, and adapted for walking, running, digging, grasping, jumping, or a myriad of other behaviors common to the individual insect. There are usually two pairs of wings, and the wings tend to be slender with veins throughout for strength. These veins and the open areas between the veins (the cells) are used to identify many insect species. While wings are common, there are a few groups of insects that evolved without wings, such as silverfish, and those that lost their wings as they specialized to feed on living animals, such as fleas. Other insects have modified wings for protection or flight. Beetles have two pairs of wings, but the first pair is hardened into a protective covering for the hind wings. Flies have one pair of well-developed wings, while the hind wings are reduced into knob-like organs that are used for balance during flight (Gullan & Cranston, 2009).

Table 5.1 Selection of insect succession studies on nonhuman animals by region

(Continued)

Basic Insect Anatomy

Table 5.1 (Continued) Selection of insect succession studies on nonhuman animals by region

The abdomen is generally cylindrical and varies from 9 to 11 obvious segments. Each segment bears a pair of holes on the lateral surfaces called the spiracles, which allow for oxygen to diffuse into the internal trachea. The abdomen does not usually have any appendages, and primarily serves to house the internal organs. The terminal end of the abdomen houses the genitalia and other specialized structures. Some female genitalia are heavily sclerotized and are used for defense as stingers (Gullan & Cranston, 2009).

These internal organs of the insect are housed within the body cavity, or the haemocoel. The haemocoel contains all the organs, including the modified heart, the brain, and the digestive, reproductive, nervous, and respiratory systems. The organs are bathed in a nutrient-rich fluid which fills the haemocoel, called hemolymph. The hemolymph combines some of the functions of vertebrate blood and lymph, delivering nutrients, removing metabolites, and carrying out basic immune functions.

Circulation of hemolymph is maintained mostly by a system of muscular pumps moving the hemolymph through compartments separated by membranes. Insects have an open circulatory system: hemolymph is not confined to vessels. Instead, it sloshes around the haemocoel and bathes the organs with nutrients and hormones while removing wastes and assisting in immune functions (Nation, 2011). Most insect hemolymph does not carry oxygen, and therefore does not have the characteristic red color that mammalian blood has. The only conducing tube is not a true vessel, but a simple dorsal tube, closed at the posterior end and dotted with openings called ostia. The ostia allow hemolymph to enter the dorsal vessel (or “heart”) and the vessel pumps the hemolymph forward toward the brain. It then flows backward in the haemocoel, assisted by muscle contractions in the ventral diaphragm (Mullen & Durden, 2002).

Oxygen is provided to the tissues via the respiratory system. Insects must obtain oxygen from the environment and eliminate carbon dioxide from the cells. Air usually enters the insect via the openings found on the lateral or posterior surfaces of the body. These openings are called spiracles and attach to a network of tracheal tubes. These tubes branch throughout the body, with the finest branches, or tracheoles, contacting all the internal organs and tissues (Mullen & Durden, 2002).

The digestive system is responsible for the breakdown of foods and the excretion of waste products. It is broken up into several sections, as follows.

The foregut receives the ingested food and is responsible for the initial digestion through muscular grinding. Chemical digestion primarily takes place in the midgut, where enzymes break down carbohydrates and proteins into absorbable units. These digestive products pass through a protective sieve that lines the midgut, the peritrophic membrane, and are absorbed through the midgut lining. Whatever is left over after absorption moves into the hindgut, which absorbs excess water and ions and excretes the waste products. Hanging off the hindgut is a variable number of tubes known as Malpighian tubules. The tubules float free in the hemolymph and filter out waste products. The waste is sent to the rectum and eliminated from the insect body (Nation, 2011).

The insect nervous system is in charge of conveying and integrating information about the internal and external environments. It consists primarily of the brain, found along the dorsal region of the head capsule, and a ventral nerve cord, which travels ventrally from the brain along the thorax and abdomen. The nerve cord is studded with ganglia, thickened regions of tissue that contain nerve cells. Each ganglion is the nerve center for its associated segment and sends information from that segment back to the brain for processing (Nation, 2011).