GENERAL ZOOLOGY 



that some insects respond differently to different wavelengths (i.e., colors) of 

 light, although the range of stimulation is not the same as that in humans. 

 Ants, for example avoid violet light, as they avoid direct sunlight, and they 

 seem not to distinguish red or orange light from darkness. Ants and honey- 

 bees are very sensitive to ultraviolet light, but humans have no conscious per- 

 ception of light in this portion of the spectrum. The structure of the com- 

 pound eye in the locust is similar to that described for the crayfish (p. 435). 

 Each eye consists of a large number of visual units, the ommatidia, each of 

 which is capable of stimulation by light from a portion of the entire visual 

 field. Eyes of this type, as previously noted, seem especially well adapted to 

 the perception of moving objects, which stimulate different ommatidia in 

 succession. 



In the locust, the tympanic membranes are assumed to be auditory organs, 

 more because of their structure than from any experimental evidence. Each 

 consists of a membrane, against the inner side of which lie structures con- 

 nected with nerves. Individual locusts react to, and so presumably "hear," 

 the rattling sound produced by the wings of other locusts in flight. Com- 

 parable flight sounds are characteristic of many other insects; the buzz of a 

 fly and the telltale whine of a mosquito are of this nature. The flight sounds 

 of mosquitoes appear to be important in mating; recordings of the sounds 

 emitted by females in flight have been shown to attract males in considerable 

 numbers. The auditory organs of mosquitoes are located on the antennae. 



The most conspicuous anatomical feature of the neurosensory mechanism 

 in insects is the degree of cephalization — the concentration of the sense organs 

 and ganglia with their adjustor neurons toward the head or cephalic end 

 of the animal. This fact is obviously correlated with the very active life 

 of most insects, which subjects them to frequent and varied changes in their 

 environments; the condition resembles the more pronounced cephalization 

 that characterizes the vertebrates. Insects exhibit reflex actions of great 

 complexity, involving what have been called chain reflexes in vertebrates. 

 Many of the activities which will be mentioned in discussing other groups of 

 insects will illustrate this statement. There is no evidence that insects have 

 any capacity comparable to the intelligence of higher vertebrates, although 

 they perform instinctively very involved actions, especially in connection with 

 mating, care of young, and colonial life. 



The Reproductive System, Reproduction, and Development. In the 

 account of external features the differences between male and female locusts 

 have been described. Such differences in external characteristics are reflected 

 in the internal details of the reproductive systems (Fig. 15.18). In the male 

 there are two testes, which lie as a saddle-shaped mass dorsal to the intestine. 

 The tubules leading from the testes pass ventrolaterally and unite into right 

 and left ductus deferentes (vasa deferentia), which join at the midline be- 

 neath the intestine to form a single ejaculatory duct traversing the penis. 

 Accessory glands, which open mto the anterior end of the ejaculatory duct, 

 secrete a fluid apparently necessary to aid the transfer of spermatozoa from 



452 



