Class Insecta 519 



theless there is a certain fundamental unity of structure that does 

 place these many forms in a single class. 



The Characteristics of the Glass. — The bodies of all insects are 

 divided into three distinct regions : the head, thorax, and abdomen. 

 Three pairs of legs are present on the thorax, and often there is one 

 or two pairs of wings. The mouth parts consists of the labrmn, the 

 mandibles, maxillae, labium, and hypopharynx. These are modified 

 among the various species depending upon their food habits ; at times 

 functional mouth parts are completely absent in the adult. 



The complete digestive tract is divided into three regions : the 

 foregut, midgut, and hindgut. The foregut is divided into the esopha- 

 gus, the large crop and the small proventriculus. The proventriculus 

 may be simply a valve, or a gastric mill may be present. The midgut 

 or ventriculus is ordinarily tubular in nature. Fingerlike diverticula, 

 the gastric caeca, arise from this region. The hindgut is composed of 

 the tubelike anterior intestine and an enlarged posterior intestine or 

 rectum. This latter connects to the anus. A pair of salivary glands is 

 present in the region of the ventriculus. Their ducts empty salivary se- 

 cretions into the mouth cavity. In butterflies and moths, these glands 

 secrete the silk which produces the cocoon ; in many blood-sucking 

 forms, the secretion possesses an anticoagulant. 



The circulatory system consists of a small tubular dorsal heart 

 and a series of blood sinuses. The blood contains various amoebo- 

 cytes and is usually colorless or greenish or yellowish. Excretion is 

 principally by means of Malpighian tubules. Respiration in most forms 

 is by means of tracheal tubes which ramify throughout the body. Some 

 aquatic forms possess gills. The nervous system is of the typical 

 arthropod type and is quite similar to that of the crustaceans with even 

 more coalescing of ganglia. 



As a group, the insects are well supplied with sense organs of 

 many different types. The head bears a pair of jointed antennae with 

 many sensory hairs. These are usually tactile in function, but may per- 

 form such diverse activities as tasting, equilibrium, and even hearing. 

 The head also has a pair of compound eyes as well as varying numbers 

 of simple eyes. The sensitivity to taste is well developed and may be 

 centered in the hypopharynx, palps, or even the tarsi of the legs. Many 

 sensory hairs are scattered over the body surface, and some insects 

 have well-developed auditory organs. Undoubtedly others possess spe- 



