244 



THE RISE OF ANIMAL LIFE 



Fig. 11-42. Insects vary a great deal in the length of 

 time they remain in the different stages in their life 

 history. The periodical cicada (Tibieina septendecim) 

 is interesting because it is thought to remain as a 

 larva in the ground for seventeen years, hence its 

 name "seventeen-year locust" (locusts are grasshop- 

 pers). Most cicadas are larvae a much shorter period 

 of time. Here is the case of one clinging to the bark 

 of a tree after the adult emerged. Note the slit along 

 the dorsal side through which it made its way. 



development of the larvae is most apt to 

 succeed. This may be in the body of another 

 insect (Fig. 11-40), the tissues of a plant, 

 the ground (Fig. 11-41), or the water. In 

 some species the eggs hatch very soon, as 

 in the housefly, while in others many weeks 

 or even months are required (Fig. 11-42). 

 Certain flies and all of the aphids bring 

 forth active young ( ovoviviparous ) . 



The eggs of some insects develop with- 

 out fertilization by a sperm. Such reproduc- 

 tion is called parthenogenesis, or unisexual 

 reproduction. This has already been ob- 

 served in bees, but it is also commonly 

 found among the aphids, where the females 

 lay eggs all through the summer months 

 which hatch only into females. As fall ap- 

 proaches, the eggs produce both males and 

 females. Fertilization then occurs and the 

 resulting eggs remain over winter and hatch 

 into females again in the spring. This proc- 

 ess seems to de-emphasize the importance 

 of males, and one begins to wonder why 

 males are necessary at all! However, they 

 do bring in the possibility of variation 

 which is impossible with only one sex. Par- 



thenogenesis is thus a regressive step, and 

 genetically is more akin to the asexual 

 budding that was observed among the coe- 

 lenterates. Obviously this is a step back- 

 wards in evolution. 



There is a wide variety among different 

 species of insects with respect to the num- 

 ber of offspring produced by one individual. 

 Some of the viviparous flies, for example, 

 produce only a few offspring, whereas the 

 queen bee may lay a million eggs in 

 her lifetime. Under optimum conditions 

 the housefly, if unchecked, could increase in 

 one summer to such proportions as to cover 

 the earth completely, for it goes through its 

 entire life cycle in eight days if the tempera- 

 ture is high enough (80-90° F.). 



The manner of development from egg to 

 adult is widely variable among many ani- 

 mals and is particularly striking among 

 the insects. The term metamorphosis, which 

 means change in form, is applied to any 

 animal that undergoes more or less marked 

 changes of form between the time of hatch- 

 ing and of reaching the adult state. A few 

 primitive insects merely increase in size 

 after hatching, showing no metamorphosis 

 (Fig. 11-43). Others, such as the grasshop- 

 per, hatch into a nymph, which resembles 

 the adult fairly closely except for the wings 

 which are acquired much later ( Fig. 11-44). 

 Such change is known as gradual metamor- 

 phosis. Some, such as the dragonfly, hatch 

 into a naiad (Fig. 11-32), which resembles 

 the adult to some extent, but not as much 

 as the nymph resembles the adult grass- 

 hopper. This type of change is called in- 

 complete metamorphosis. In the case of the 

 housefly or June beetle, the larval stage 

 does not resemble the adult in any way 

 (Fig. 11-45); the larva is worm-like and 

 usually its diet varies radically from that of 

 the adult. Moreover, between the larval and 

 the adult stage there is a "resting" stage, 

 known as the pupa, during which time the 

 larval body is transformed into the adult 

 body. This type of change is known as 

 complete metamorphosis. Other aspects of 



