X1Y INTRODUCTION. 



fication does not occur on the secondaries, and all the branches 

 terminate in the outer margin ; so also with the branches of the 

 posterior median of the primaries. On the other hand, the two 

 first branches on the primaries end on the anterior margin, nerves 

 11 and 10; the third proceeds as nerve 6 to the outer margin, but 

 at or behind the cross nerve it furcates, and nerve 8, which here 

 arises out of it, proceeds usually to the apex or near it, after it has 

 again branched off, and nerve 7 is sent off to the outer margin and 

 nerve 9 to the anterior, fig. 24. Nerve 10 also frequently furcates, 

 in sending one branch towards the outer margin, impinging on 

 nerve 8, usually at the place where nerve 7 originates, so that this 

 is the continuation of that branch, which then cuts nerve 8. There 

 are some deviations from this structure, especially in the micro- 

 lepidoptera, of which the neuration of the primaries is more simple. 

 It is not to be denied that the anterior margin and the apex of the 

 primaries, on which the power of sustaining flight chiefly depends, 

 acquire a greater strength from the complicated nerve structure, 

 whilst the secondaries do not require it, inasmuch as they are closely 

 united with the primaries ; but on the other hand, on the interior 

 margin they are furnished with more simple dorsal nerves, which 

 are sufficient to sustain the wider superficies of the wings. The 

 dorsal nerve of the primaries and the costal of the secondaries also 

 appear to bear a mutual relation to each other, and this may also 

 be true with regard to the corresponding margins. Both these 

 nerves often have an inclination to furcate toward the root of the 

 wings. .In those cases, for instance, where the costa3 of the infe- 

 riors after its origin unites with the anterior median, it is essentially 

 a furcation, in which case, however, the one branch constitutes part 

 of the median, or coincides with it. 



The spaces between the nerves are called cells, cellulce, and are 

 so distinguished by numbers, that the cell always contains the 

 number of the nerve which it follows, counted from the interior 

 margin. Thus the cell between the nerves 2 and 3 is called cell 2; 

 the one between the nerves 3 and 4 is called cell 3, &c. &c. In 

 fig. 25, they are distinguished by Roman letters. The cells, on 

 the contrary, between the interior margin and nerve 2 are distin- 

 guished as cell la, Ib, Ic, Id, fig. 26. The cell between the cross 

 nerve and the two medians is called the median cell, cellula media, 

 cm. Occasionally it is divided by one or two longitudinal nerves, 

 in which case the distinct parts are called the anterior median cell 



