248 W. WESCHE ON MODIFICATIONS OF LEGS IN DIPTEROUS FLIES. 



arrangement of spines and the alterations in the shape of the 

 Hmb may be compared with the middle leg of Campsicnemus 

 (Fig. 10). We here see, in two distantly-related species, the 

 identical causes, working on parallel lines to approximately the 

 same results. 



We now come to a case in which the limbs have been 

 strikingly modified, but from obscure causes. Speaking of the 

 Crane-fly, Tijnda oleracea (the familiar " daddy longlegs "), Dr. 

 Sharp, in the "' Cambridge Natural History," says, " It is 

 impossible to assign any reason of utility for the extreme 

 length of the legs of crane-flies : they break off with extreme 

 ease, and the insect appears to get on perfectly well without 

 them." I do not think the problem here presented is of such 

 difiiculty as that of the use of the hind legs of some of the 

 Syrpliidae. The eggs of crane-flies are laid on grassland, and the 

 larvae, as is well known, do inj ury to the roots. Tt is conceivable 

 that the extreme length of limb is useful to the insect in making 

 its way through short herbage. Such being the case, we would 

 expect the male to develop longer limbs than the female, as 

 that would be of advantage in finding and catching her, and in 

 transmitting that characteristic to his offspring. We find that 

 males frequently have longer limbs than females — a fact which, if 

 not proving, at all events suggests, that length of limb has been 

 brought about by the law of " survival of the fittest." An 

 analogous case is the eye, which is often larger in the males 

 than in the females of Diptera. It may therefore be conceded, 

 on this evidence, that length of limb is useful. It is obvious that 

 much increase in weight in an insect creeping from grass-blade 

 to grass-blade would not be of service, therefore the increase of 

 length was not accompanied by an increase of strength. It is 

 further conceivable that the frequent loss of limbs would produce 

 a race which would bear that loss with a certain amount of 

 immunity. The hardy male or female who could lose a joint or 

 so without succumbing would have a better chance of transmitting 

 that hardihood to his or her offspring. It has been suggested 

 that the length of limb would be of service as a balance when 

 flying, by counteracting the weight of the long abdomen. That 

 being so, though the crane-fly does not use his wings as much 

 as his legs, it would further aid the increase of the length of the 

 legs. Darwin says that the males of Tipidae frequently fight 



