282 



METAZOAN PHYLA 



Poilpi 

 Mandible 



nfenna 



Esophagus 

 Crop 



Pro venfri cuius 

 Venfriculus 



lan 



The alimentary canal is modified according to the character of the 

 food. A mandibulate insect (Fig. 187) usually possesses an esophagus, 

 which may be dilated posteriorly to form a crop; a muscular gizzard, or 

 proventriculus, which grinds the food and also strains it; a digestive 

 stomach or ventriculus, which receives the secretion from a number of 

 gastric glands, or caeca; and an intestine, which receives the digested 

 food and into which also open the tubular organs of elimination, the 

 malpighian tuhules. Suctorial insects do not have a gizzard, but in place 



of it they have a muscular pharynx 

 which acts as a pump and a sac for 

 the storage of juices (Fig. 186). 



The nervous system of insects is 

 similar in general plan to that of the 

 earthworm, but there are two ventral 

 nerve cords (Fig. 186). The two gan- 

 glia of each pair are usually fused and 

 communicate by commissures. In 

 the lower insects there is a pair of 

 ganglia to each segment, but in 

 the higher forms the number is 

 reduced. In the latter forms the 

 thoracic ganglia are increased in size 

 and the supraesophageal and sub- 

 esophageal ganglia are not only 

 increased in size but tend to be 

 brought together by the shortening of 

 the circumesophageal connectives. 

 This results in such a degree of 

 centrahzation and cephalization that 

 these anterior ganglia may properly 

 be called a hrain (Figs. 186 and 188). 

 Their removal interferes with the 

 coordination of movements and 

 results in death, though this is not immediate. 



314. Senses of Insects. — Insects possess a great variety of sense 

 organs and several may serve for the reception of the same general 

 type of stimulus. The compound eyes and ocelh are both organs of 

 sight, although the exact function of the latter is not well understood. 

 The compound eye of an insect is similar to that of the crayfish (Fig. 167). 

 The sense of smell is highly developed in insects though olfactory organs 

 are known to exist only on the antennae. On the palpi and about the 

 mouth are organs of taste. Tactile organs are located on the antennae 

 and elsewhere about the body. That insects have a sense of hearing is 

 indicated by the variety of sounds they produce. Sometimes these are 



Anal 

 glands 



Fig. 187. — Digestive system of a 

 beetle, Carahus aurahis Linnaeus, as an 

 example of a mandibulate insect. 

 {From Lang, "Text-book of Comparative 

 Anatomy," after Dufour.) 



