INSECTA. 247 



dissimilar kinds. In proof of this, he adduces the fact, that in the 

 same family we not unfrequently meet with both winged and ap- 

 terous species nearly related to each other ; and in many cases the 

 males possess wings, while the females of the same insect are en- 

 tirely destitute of such appendages. In such cases, the metamor- 

 phosis is necessarily what is called incomplete, inasmuch as the 

 organs which characterize the perfect state are not developed. Thus, 

 in the flea (Pulex irritans) (fig. 110), the wings never become 

 apparent, and the Fig. 110. 



thorax in conse- 

 quence, even in 

 the imago state, 

 does not exhibit 

 that develope- 

 ment and con- 

 solidation of its 

 parts invariably 

 met with in wing- 

 ed genera. The flea, however, cannot on this account be looked 

 upon as any other than the imago or complete insect, for it will 

 be found to have undergone all the preparatory changes. The 

 flea, when it issues from the egg, is in fact a worm-like and foot- 

 less larva, in which condition it lives about twelve days. When 

 about to become a pupa, it spins for itself a little silky cocoon, 

 wherein it conceals itself, until, having thrown off its last skin, 

 it appears in its mature form, deprived indeed of wings, that, under 

 the circumstances in which it lives, would be useless appendages, 

 but still with this exception corresponding in every particular with 

 other insects in their imago state. 



(288.) The wings of insects differ much in texture. In the Neu- 

 roptera, by far the most powerful fliers met with in the insect 

 world, all four wings are of equal size, and consist of a thin mem- 

 branous expansion of great delicacy and of a glassy appearance, 

 supported at all points by a horny network (Jig. 103). Few 

 things are met with in nature more admirable than these struc- 

 tures ; they present indeed a combination of strength and lightness 

 absolutely unequalled by anything of human invention, and as 

 instruments of flight they far surpass the wings of birds, both 

 in the power and precision of their movements, inasmuch as these 

 insects can fly in all directions, backwards, or to the right or left, 

 as well as forwards. Leeuwenhoek* narrates a remarkable instance 

 * Leeuw. Epist. 6, Mart. 1717. 



