390 PHYLUM ARTHROPODA 



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 auto- 

 matic 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. 



In the case of hive-bees there is evidence of a power of "homing." 

 They return straight to the hive from a distance of over a mile, even 

 when they have been carried afield in a box. In great measure they 

 learn the features of their district. 



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 terinites gnaw decaying 

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

 carum," Linnaeus called them), whose sweet juices they 

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

 vegetables in which they are hatched. 



Many modifications of mouth organs, and of the alimen- 

 tary canal, are associated with the way in which the insect 

 feeds. 



The alimentary canal consists of fore-gut, mid-gut, and 

 hind-gut, but in many cases it seems very doubtful if the 

 mid-gut has its typically endodermic character. 



The fore-gut conducts food, and includes mouth cavity, 

 pharynx, and cesophagus, the latter being often swollen into 

 a storing crop, or continued into a muscular gizzard with 

 grinding plates of chitin. 



The mid-gut is digestive and absorptive, often bear- 

 ing a number of glandular outgrowths or caeca, and 

 varies in length (in beetles at least) in inverse pro- 

 portion to the nutritive and digestible quality of the 

 food. 



The hind-gut is said to be partly absorptive, but is 

 chiefly a conducting intestine, often coiled and terminally 

 expanded into a rectum with which glands are frequently 

 associated. 



