. Animal biology. Zoology; Biology. 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
. Animal biology. Zoology; Biology. 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 i
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