344 PHLYUM ARTHROPODA. 



one lens (monomeniscous), whereas the compound forms 

 have many lenses (polymeniscous). In the simple eye each 

 retinal unit is a single cell, of which the distal part is unpig- 

 mented. In the compound eye the retinal unit consists 

 of six cells around an axis. The structure of ocelli varies 

 greatly, and their use is very uncertain. 



Auditory (or chordotonal) organs have been found in all orders of 

 Insects (except as yet the Thysanoptera), and occur both in the larvae 

 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, antennae, 

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

 of the abdomen. 5J 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. 



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 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. 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 hea'd 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 definite evidence of a sense of direc- 

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

 even when they have been blinded and robbed of antennae, even when 

 they have been carried afield in a box. 



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," Linnaeus called them), whose sweet juices they 



