Page:Insects - Their Ways and Means of Living.djvu/152



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FIG. 7 2. The general nervous system of a grass- hopper, as seen from above /lnt, antenna; /o, aorta;/i'r, brain; Cer, cercus; E, compound eye; Gngl, 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; Gngll/oEngl/llI, ganglia of the fourth to eighth abdominal seg- ments; O, ocelli; Proc, proctodeum, or posterior part of alimentary canal; .ça, suranal plate; .çeglI--X, second to tenth segments of abdomen; .çoeGng, suboesophageal ganglion; .çtom, stomo- deum, or anterior part of alimcntary canal

stimulus, most like- ly, comes from the products of physio- logical changes be- ginning 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 ignor- ance of the processes that cause them. External stimuli are things of the outer environment that affect the living organism. They in- clude matter, elec- tromagnetic 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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