278 



METAZOAN PHYLA 



the result of rubbing rough surfaces together. The locust rubs the 

 femora of the hind legs against the outer surfaces of the tegmina or 

 produces the crackling sound heard in flight by rubbing the front edges 

 of the hind wings against the tegmina. The shrilling of a cicada is due 

 to the vibration of a stiff chitinous membrane drawn across a sound 

 chamber; this vibration is controlled by muscles. The buzzing of many 

 insects is due to rapid vibration of the wings. The auditory organ of a 

 locust is a pit, or tympanum, on the first abdominal metamere, closed 

 by a thin kidney-shaped tympanic membrane (Fig. 170). Organs 

 believed to be auditory also exist on the antennae of many insects and on 



ABC 



Fig. 177. — Nervous systems of three insects to illustrate condensation and cephaliza- 

 tion. A, a termite, one of the lowest insects. S, a water beetle. C, a fly, the highest type. 

 (From VanCleave, "Invertebrate Zoology," A after Lespes, B and C after Blanchard, by the 

 courtesy of McGraw-Hill Book Company, Inc.) 



the legs of katydids. A few insects, especially the larvae and adults of 

 certain beetles known respectively as glowworms and fireflies, are lumi- 

 nescent. The emission of light, like the utterance of sounds, probably 

 serves to bring the sexes together. 



313. Reproduction. — Insects are always diecious. The eggs, which 

 are fertihzed internally, undergo superficial cleavage and develop much 

 as do those of the crayfish. From the egg hatches a larva which differs 

 in character in the various groups. Most of the insects exhibit a meta- 

 morphosis. Only a few do not do so, and these are termed ametabolous 

 (Fig. 178) and are grouped together as Ametabola (am e tab' o la; G., 

 ametaholos, unchangeable). Most of them pass through several stages, 

 but the number of stages varies. In all insects the larval form is the 

 period during which growth takes place, no insect growing after it has 

 become adult. 



