INSECTS
Fig. 72. The general nervous system of a grasshopper, as seen from above
Ant, antenna; Ao, aorta; Br, brain; Cer, cercus; E,compound eye; Gng1, ganglion of prothorax; Gng2, ganglion of mesothorax; Gng3+I+II+III, compound ganglion of metathorax, comprising the ganglia belonging to the metathorax and the first three abdominal segments; GngIV–GngVIII ganglia of the fourth to eighth abdominal segments; O, ocelli; Proc, proctodeum, or posterior part of alimentary canal; Sa, suranal plate; SegII–X, second to tenth segments of abdomen; SoeGng, suboesophageal ganglion; Stom, stomodeum, or anterior part of alimentary canal
stimulus, most likely, comes from the products of physiological changes beginning to take place in the body that will soon result in the transformation of the caterpillar into a chrysalis, a stage when the insect needs the protection of a cocoon. These activities of insects we call instincts, but the term is simply a cover for our ignorance of the processes that cause them.
External stimuli are things of the outer environment that affect the living organism. They include matter, electromagnetic energy, and gravity; but the known stimuli do not comprise all the activities of matter or of the "ether." The common stimuli are: pressure of solids, liquids, and gases; humidity; chemical qualities (odors and tastes);
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