328 PHYLUM ARTHROPOD A. 



and in the adults. Their essential structure is as follows : A nerve ends 

 in 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, antennce, 

 palps, legs, wings, in the halteres of Diptera, and upon the dorsal aspect 

 of the abdomen." 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 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 antennas and mouth-parts of many 

 insects. Not a few insects seem 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 

 emotions. 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 

 Hymenoptera ; 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 its 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 serve as alluring 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 carnivorous (we need not specify the 

 homceopathist'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 

 wood; some ants keep aphides as cows ("vaccae formi- 

 carum," Linn?eus called them), whose sweet juices they 

 lick ; and a great number of larvae devour the flesh and 

 vegetables in which they are hatched. 



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 useful : 



