344 PHLVYUM 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 recinal unit consists 
of six cells around an axis. The stricture 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 larve 
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, antennz, 
palps, legs, wings, in the halteres of Diptera, and upon the dorsal aspect 
oftheabdomen.” 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 antenne 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 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 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 antenne, 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 (‘“vacce formi- 
carum,” Linnzeus called them), whose sweet juices they 
