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Lecture 2: Insect Morphology
Morphology: The study of form and function Insects are arthropods: General characteristics of arthropods: General characteristics of insects: A look at the outside of an insect: The exoskeleton is comprised of sclerites: hardened plates The integument (body covering) is comprised of multiple layers: The insect head bears: mouthparts, eyes, and antennae. Mouthparts: Mouthparts may be modified greatly from the "generalized" plan ... see illustrations of the cicada and the house fly in comparison with the general form exhibited by the grasshopper.
The orientation of the mouthparts on the head may differ, and they may be described as: Prognathous: projecting forward (horizontal) Hypognathous: projecting downward Opisthognathous: projecting obliquely or posteriorly
Eyes: Compound eyes: Individual units are facets or ommatidia. 28,000 ommatidia comprise a single compound eye in dragonflies Oellus (Ocelli), or simple eyes: small, usually a single lens Antennae: 2 basal segments are the scape & pedicel The filament is comprised of several segments (actually pseudo-segments lacking independent musculature) a. setaceous: hair-like
The insect thorax: 3 distinct segments: Sclerites that comprise the thorax are given specific names; each may be preceded by the prefixes pro-, meso-, or meta-. Legs are segmented. The names for each segment are (in order, beginning at the body and progressing outward.
The tarsus may be comprised of multiple segments (not really true segments; more accurately called tarsomeres); the terminal segment usually bears claws. Legs may be modified for specific purposes: Wings Wing veins and cells between veins are named according to the standard system illustrated below:
Wing modifications: The insect abdomen ... (Homologous = structures with similar evolutionary origin but different function, such as the different forms of mandibles in insects. Analogous = structures with similar functions but different evolutionary origins, such as the wings in birds versus insects.) Inside the insect:Digestive System: A tube that extends from the mouth to the anus; there are3 sections: Foregut: Midgut: Hindgut: In embryonic development, the foregut and hindgut are formed from ectoderm ... their surfaces are shed during molting. The midgut is formed from the endoderm; its surface is retained during molting.
Digestion: Some insects use external digestion in addition that which occurs internally ... Most digestive action is in the midgut; gastric caecae are rich in enzymes. Enzyme diversity varies with the range of foods that different insects eat ... more proteolytic enzymes in blood feeders, cellulase in wood-boring beetles, etc. Excretory system: Purpose of excretion: Primary excretory organs: Malpighian tubules and the rectum. Malpighian tubules "float" in the hemolymph; active transport moves wastes (uric acid salts from the fat body) into tubules. Malpighian tubules empty into the hind gut; water is reabsorbed. Excretory and fecal wastes are combined. Nervous system: The brain = the supraesophageal ganglion (nerve cell mass above the esophagus)
The ventral nerve cord: connects segmental ganglia (nerve cell bundles). Thoracic and abdominal ganglia control many body operations. The corpora cardiaca and corpora allata are neuroendocrine glands. Lecture 3 will include more on neurohormones and their function. Chemoreceptors (taste and smell) take the form of sensory pegs on various body structures, particularly antennae, tarsi, and palpi. Photoreceptors are located in the compound eyes and the ocelli (and also the cuticle). Hearing organs may be located on the abdomen (grasshoppers), tibiae (crickets), or thorax (moths). Respiratory system (tracheal system):
Special modifications: Circulatory system: Insects have what is termed an "open" circulatory system. It is comprised of a dorsal vessel with a posterior "heart" and an anterior aorta. The heart pumps blood (hemolymph) forward and empties it over the brain. Blood percolates backwards. Specialized pulsating organs in some insects contribute to blood flow, including flow through wing veins. The role of blood in insects is the transport of nutrients, wastes, and hormones. It is NOT the primary means of moving oxygen and carbon dioxide. (There is no hemoglobin in insects except in immature Chironomus spp. and a few others.) Reproductive system: Structures are named by similar terms as those in vertebrates. Key differences:
Musculature: Muscles and flight: Direct musculature is involved in the flight of more primitive insects (for example, the Odonata); but in more advanced fliers, only indirect flight muscles are used. Indirect muscle contractions and relaxations move the notum up and down, and thoracic pleurites serve as a fulcrum. This mechanism allows phenomenal wingbeat frequencies ... 1,000 beats per second. For more information ... Check the web site. "An Introduction to Insect Anatomy" at the Wonderful World of Insects. See pages 5-25 in the textbook, Photographic Atlas of Entomology and Guide to Insect Identification, by J.L. Castner, Feline Press, Gainesville, FL (2000). |
Introduction to Applied Entomology If you find any problems with this page, please notify Rick Weinzierl, weinzier@uiuc.edu. |
