ANIMAL FLIGHT 121 



But on the other hand the habit of hanging by the tail and 

 of using the tail as an organ of prehension and of locomotion 

 is almost exclusively confined to tropical America where it is 

 characteristic of many animals in many very diverse groups, 

 as monkeys, carnivorous animals, opossums, rats and porcupines. 

 Why should this be so? 



Let us now briefly survey the insects, the most numerous 

 by far of all the flying creatures. In their younger stages all 

 insects are wingless, but when adult most insects can fly. Of 

 all of them the tsetses and some of the bird flies fly the longest 

 in proportion to their length of life. These flies are born as 

 pupae or as larvae just ready to transform to pupae from which 

 adults emerge. They do not feed as larvae or as pupae, and 

 their adult winged existence is correspondingly prolonged. 



In many insects the flying stage is very short. For instance 

 the seventeen year locust, or "periodical cicada" as the en- 

 tomologists would prefer to have us call it, spends only about 

 one nine hundredth part of its existence in the winged state, 

 and it does not fly much even in that short time; whfle in 

 some may-flies, which lack a mouth and therefore cannot feed, 

 the flying period is less than one one thousandth part of their 

 whole life. Thus if we were may-flies flying would be possible 

 for not more than twenty-five days out of a normal life. 



In most insects the flying stage is rather short compared with 

 the whole length of life, and in very few is it so much as a 

 quarter of their whole existence. Also, in most flying insects 

 both sexes fly equally well, as among the birds and bats, but 

 in many the larger and heavier females are much less expert 

 than the males, and in some the females cannot fly at ah, the 

 wings being much reduced in size or even absent altogether. 



Let us here repeat that the relative size of an insect's wing is 

 much greater than that of a bird's wing. An insect is so light 

 that it has no momentum, so that the wings must continually 

 pull the body forward as well as lift it. Since there is no 

 momentum the lifting and the pulling must be as nearly con- 

 tinuous as possible, so that the wing motion of insects is 



