304 INSECTA. 



as well as of a broad fringe of stiff hairs inserted all around the tarsus. 

 The powerful oars thus formed can open until they form right angles 

 with the axis of the body, and from the strength of their stroke are well 

 adapted to the piratical habits of their possessors, who wage successful 

 war, not only with other aquatic insects and worms, but even with small 

 fishes, the co-inhabitants of the ponds wherein they live. 



(792.) The same principles are carried out even more perfectly in 

 the construction of the swimming-legs of the Water -boatman (Notor- 

 necta), a kind of water-bug. The resemblance of this creature (fig. 144, 

 G, H) to a boat with its oars cannot escape the most inattentive exa- 

 miner ; and the similarity is still further increased by its manner of 

 swimming ; for, as it preys upon insects that have been accidentally 

 drowned by falling into the water, it usually rows itself about upon its 

 back, because in such a position it can best watch for its victims. 



(793.) The wings of insects, when present, are invariably attached 

 to the two posterior segments of the thorax, which, as we have already 

 seen, are strengthened in every possible manner, so as to afford a sup- 

 port of sufficient density and firmness to sustain the violent exertions 

 of the muscles inserted into the organs of flight. 



(794.) In the most perfectly organized families the wings are four in 

 number, as in the Neuroptera (fig. 146), the Hymenoptera (fig. 173), 

 the Orthoptera (fig. 145), the Dictyoptera, the Hemiptera (fig. 144), 

 the Lepidoptera (fig. 148), and the Coleoptera (fig. 149). 



(795.) In the Dipterous insects there are only two wings, which are 

 fixed upon the central segment of the thorax ; while, in the position 

 usually occupied by the posterior pair, we find a pair of pedunculated 

 globular bodies, generally named the halteres or poisers, as in the Gnat 

 (CWaO (fig. 177, F). 



(796.) But, in every one of the orders above enumerated, there are 

 certain families which, throughout the whole period of their existence, 

 are never provided with wings at all ; and these by many entomologists 

 have been formed into an order by themselves, under the name of Apte- 

 rous insects. In the opinion of Burmeister*, whose classification we 

 have adopted, such an arrangement is purely artificial, inasmuch as it 

 must embrace insects of most dissimilar kinds. In proof of this, he ad- 

 duces the fact that, in the same family, we not unfrequently meet with 

 both winged and apterous species, nearly related to each other ; and in 

 many cases the males possess wings, while the females of the same insect 

 are entirely 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. 153) the wings never become apparent, and 

 in consequence the thorax, even in the imago state, does not exhibit 

 that development and consolidation of its parts invariably met with in 

 * Manual of Entom. p. 623. 



