312 PERIPATUS, MYRIOPODS, AND INSECTS. 



a centre or ganglion near the skin, some of the cells of this ganglion grow 

 out into long sensitive rods enclosed in a tiny sheath, the rods are directly 

 or indirectly connected with the epidermis above them. "They are found 

 in groups of 2-200 in various parts of the body, antennre, palps, legs, 

 wings, in the halteres of Diptera, and upon the dorsal aspect of the abdo- 

 men." Quite different from these, and occurring in flies alone, on the 

 hind end of the larva, or at the base of the adult's feelers, are little bags 

 with fluid in which clear globules float. We do not know how much or 

 how little Insects hear, but the " song " of male Cicadas and crickets 

 does not fall on deaf ears. 



In addition to the "eyes" and "ears" there are innervated hairs 

 (tactile, tasting, olfactory) on the antennae and mouth parts of many 

 insects. Not a few have been shown to possess a diffuse or dermatoptic 

 sense, by which, for instance, they can, when blinded, find their way 

 out of a dark box. 



Many Insects produce sounds which often express a variety of emo- 

 tions. We hear the whirr of rapidly moving wings in flies, the buzz of 

 leaf-like structures near the openings of the air-tubes in many Hymenop- 

 tera, the scraping of legs against wing ribs in grasshoppers, the chirping 

 of male crickets which rub one wing against its neighbour, the piping of 

 male Cicadas which have a complex musical instrument, the voice of the 

 death's-head moth which expels air forcibly from its mouth. The death 

 watch taps with his head on wooden objects, as if knocking at the door 

 behind which his mate may be hidden. In some cases the sounds are 

 simply automatic reflexes of activity ; in many cases they ser\'e as allur- 

 ing love calls, and they may also serve as expressions of fear and anger, 

 or as warning alarms. 



Alimentary System. 



The diet of Insects is very varied. Some, such as locusts, 

 are vegetarian, and destroy our crops ; others are carnivor- 

 ous (we need not specify the homo^opathist's leech) and 

 suck the blood of living victims, or devour the dead ; the 

 bees flit in search of nectar from flower to flower, while the 

 ant lion lurks in his pit of sand for any unwary stumbler ; the 

 termites gnaw decaying vi'ood ; some ants keep aphides as 

 cows ("vaccfe formicarum," Linnteus called them), whose 

 sweet juices they lick ; and a great number of larvae devour 

 the flesh and vegetables in which they are born. 



It is important to have some vivid idea of the diversity of 

 diet, for the many modifications of mouth organs, in beetle 

 and bee, in caterpillar and butterfly, as well as differences in 

 the alimentary canal itself, are associated with the way in 

 which the insect feeds. 



For purposes of classification, the following distinctions in regard to 

 the mouth organs are very useful : — 



