242 



THE RISE OF ANIMAL LIFE 



siderably, depending on their type of diet. 

 Furthermore, the diet may differ widely 

 during the larval and adult stages : the but- 

 terfly, for instance, feeds on leafy vegeta- 

 tion as a larva, and on nectar as an adult. 

 Feeding may be confined to certain stages 

 of the life cycle and absent in others. Such 

 insects as the May flies and fishflies feed 

 only as larval forms, the adults living but 

 a few days during which time food is un- 

 necessary. The adult stage is devoted to 

 mating and egg-laying. Among mosquitoes, 

 the males mate and die, never feeding at 

 any time on blood, whereas the females of 

 some species must obtain a blood meal be- 

 fore her eggs will mature. She has a vora- 

 cious appetite and is able to take a meal 

 of blood equal to several times her body 

 weight. 



Some insects, such as certain species of 

 termites, feed entirely on wood, a carbo- 

 hydrate, apparently never taking in any 

 nitrogen. They are able to exist on this diet 

 because of the action of the Protozoa ( flag- 

 ellates ) which inhabit their intestinal tract. 

 Neither the Protozoa nor the tennites can 

 survive without the other, a case of perfect 

 mutualism, as was pointed out earlier ( Fig. 

 5-9). 



Some insects, like the adult dragonfly, are 

 carnivores (meat eaters), others, like the 

 grasshopper, are herbivores (vegetable eat- 

 ers), and still others, like the cockroach, are 

 omnivores (both meat and vegetable eat- 

 ers). However, almost any one of them 

 can be forced to change its usual dietary 

 habits when it is confronted with starvation. 



The sense organs and nervous systems 

 have become greatly modified among the 

 insects. The central nervous system of some 

 of the lower insects does not differ greatly 

 from tliat found in the earthworm, while in 

 others, like the honeybee, there has been 

 a great deal of fusion of ganglia and an ap- 

 parently higher or more closely knit coor- 

 dination of parts developed. 



Insects have developed better means of 

 communication than is found among the 



lower forms. Their ability to produce and to 

 hear sounds has already been described 

 in some forms. Another means of communi- 

 cation is illustrated by the firefly, which is 

 able to produce a light which seems to bring 

 the sexes together at the mating season. 

 How this light is produced is an interesting 

 problem, and one which biologists have 

 studied for a lono; time. It will be discussed 

 under the topic of bioluminescence. Most 

 insects are remarkably sensitive to chemi- 

 cals, especially in the air, greatly exceeding 

 man in this respect. Some leave a faint scent 

 which is detected by other members of the 

 species, usually of the opposite sex. It is a 

 common schoolroom experiment to place 

 a female Cecropia moth, as she emerges in 

 the spring, on the inside of a window screen. 

 Very shortly a great many males, detecting 

 her presence either by odors or by the pro- 

 duction of sound, will collect on the outside 

 of the screen. The male mosquito has a very 

 large feathery antenna with which he can 

 detect aerial vibrations coming from the 

 female as much as a quarter of a mile away. 

 The social insects represent a very high 

 development of the nervous system, per- 

 haps the highest in the invertebrates. There 

 is a long series of gradations from the soli- 

 tary insects to those which aggregate during 

 hibernation or migration, like some grass- 

 hoppers and the monarch butterfly. Out of 

 something like this gregarious behavior may 

 have come the parental care in guarding 

 the eggs and later the young. From such 

 species have come the ti-ue social forms 

 which live together in lar2;e numbers and 

 have developed various castes, as already 

 described in the honeybee. The change in 

 the various castes has been fundamental 

 because it involves hormonal changes 

 which in turn alter the anatomy of the 

 caste, as, for example, the development of 

 the large mandibles of certain of the sol- 

 diers among some species of termites. In 

 this group, in addition to the soldiers which 

 protect the colony, there are the workers, 

 the males, and the queen. In some ant colo- 



