314 ANIMAL MOVEMENT 



long slender abdomen the tip of this is turned over the back to 

 assist in stowing the membranous wings under their covers. 



There are some curious little forms (species of Sty lops, &c.), 

 usually regarded either as aberrant beetles or as the representatives 

 of a distinct order (Strepsiptera), which give us the exact opposite 

 of the arrangement described for two-winged flies. They are 

 parasitic in bees and some other insects, and the females are like 

 minute maggots in appearance. The males, however, can fly with 

 great activity by means of very large hind-wings, while the fore- 

 wings are only represented by little scales, which are probably the 

 last remains of well-developed wing-covers once possessed by 

 ancestral forms. 



Some insects have given up flight altogether, and lost both 

 pairs of wings; such as Fleas, in which some little scales on the 

 sides of the thorax are commonly regarded as the vestiges of the 

 wino's, and Lice, in which there is no trace of wing's at all. In 

 the forms mentioned in the last paragraph the females only have 

 become flightless, and they are by no means the only illustration 

 of this phenomenon that could be given. The Primitive Wing- 

 less Insects (Aptera) probably resemble in many ways the early 

 ancestors of the group, which were not provided with organs of 

 flight. 



Evolution of the Wings of Insects. — There have been many 

 speculations as to the early stages in the evolution of the wings of 

 insects, and it is obvious that at their first inception they cannot 

 have been flying organs. The most plausible suggestion is the 

 one that has already been made for birds and bats, i.e. that the 

 structures which have been gradually specialized into wings arose 

 in the first instance as parachute-folds, in this case by outgrowth 

 from the sides of the thorax. Possibly such folds were first de- 

 veloped as an aid to leaping, in which case comparison may be 

 made with the Australian Flying-Spider (see p. 289). But, on the 

 whole, it is more likely that the development of parachute-folds 

 was a sequel to climbing, and this view becomes more probable 

 when we recollect that the number of legs which insects possess, 

 i.e. six, is best explained by reference to the climbing habit. The 

 establishment of joints between the folds and the thorax would be 

 of service in guiding even parachute movements, and from this 

 stage on it is not difficult to imagine the gradual modification of 

 the folds into wings. 



