SENSES OF INSECTS 389 



degree concentrated, and in the adults are usually more 

 centralised than in the larvae. 



Sensory structures. — Animals so much alive as Insects, 

 and in surroundings so stimulating as many of them enjoy, 

 have naturally highly developed sense organs. 



Two compound eyes are present on the head of all adults 

 except the primitive CoUembola, the degenerate lice, the 

 likewise parasitic fleas, and bhnd insects which live in caves 

 or other dark places. Each eye contains a large number of 

 similar elements, in each of which we can distinguish — 

 (i) a cuticular or corneal facet ; (2) a glassy lens-like 

 portion ; (3) a retinal portion in association with which 

 are fibres from the optic nerve ; and there are also pig- 

 mented cells between the elements. 



In addition to the compound eyes, simple eyes or oceUi 

 are present in the adults of many insects, e.g. ants, bees, 

 and wasps ; they occur without the accompaniment of com- 

 pound eyes in CoUembola, lice, and fleas, and they are 

 usually the only eyes possessed by larvae. They have only 

 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 un- 

 pigmented. 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. Thek 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." 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 httle 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 



