<P> The digestive system of beetles is primarily based on plants, which they for the most part feed upon, with mostly the anterior midgut performing digestion . However, in predatory species (e.g., Carabidae) most digestion occurs in the crop by means of midgut enzymes . In Elateridae species, the predatory larvae defecate enzymes on their prey, with digestion being extraorally . The alimentary canal basically comprises a short narrow pharynx, a widened expansion, the crop and a poorly developed gizzard . After there is a midgut, that varies in dimensions between species, with a large amount of cecum, with a hingut, with varying lengths . There are typically four to six Malpighian tubules . </P> <P> The nervous system in beetles contains all the types found in insects, varying between different species . With three thoracic and seven or eight abdominal ganglia can be distinguished to that in which all the thoracic and abdominal ganglia are fused to form a composite structure . Oxygen is obtained via a tracheal system . Air enters a series of tubes along the body through openings called spiracles, and is then taken into increasingly finer fibers . Pumping movements of the body force the air through the system . Some species of diving beetles (Dytiscidae) carry a bubble of air with them whenever they dive beneath the water surface . This bubble may be held under the elytra or it may be trapped against the body using specialized hairs . The bubble usually covers one or more spiracles so the insect can breathe air from the bubble while submerged . An air bubble provides an insect with only a short - term supply of oxygen, but thanks to its unique physical properties, oxygen will diffuse into the bubble and displacing the nitrogen, called passive diffusion, however the volume of the bubble eventually diminishes and the beetle will have to return to the surface . </P> <P> Like other insect species, beetles have hemolymph instead of blood . The open circulatory system of the beetle is driven by a tube - like heart attached to the top inside of the thorax . </P> <P> Different glands specialize for different pheromones produced for finding mates . Pheromones from species of Rutelinea are produced from epithelial cells lining the inner surface of the apical abdominal segments or amino acid based pheromones of Melolonthinae from eversible glands on the abdominal apex . Other species produce different types of pheromones . Dermestids produce esters, and species of Elateridae produce fatty - acid - derived aldehydes and acetates . For means of finding a mate also, fireflies (Lampyridae) utilized modified fat body cells with transparent surfaces backed with reflective uric acid crystals to biosynthetically produce light, or bioluminescence . The light produce is highly efficient, as it is produced by oxidation of luciferin by the enzymes luciferase in the presence of ATP (adenosine triphospate) and oxygen, producing oxyluciferin, carbon dioxide, and light . </P>

What is the unit structure of the body covering of an insect