16 ME. A. HAMMOND ON THE 



2nd. The evidence derivable from developmental change ; 

 3rd. That obtainable from a consideration of the nervous and 

 muscular systems. 



Considerations of Analogies in divers Insects. — I may observe 

 that the three orders of hexapod insects associated by Packard* 

 under the name of Metabola, viz. the Lepidoptera, the Hymeno- 

 ptera, and the Diptera, beside the point of resemblance pointed 

 out by him, have this in common, viz. the excessive development 

 of the mesothorax at the expense of the preceding and following 

 segments. That this is broadly the case is, I believe, an admitted 

 fact irrespective of questions at present under discussion ; and 

 it will be worth our while to consider what relation this prepon- 

 derance of the mesothoracic over at least the metathoracic region 

 bears to the development of the wings and to their effectiveness 

 as organs of flight. Of the three orders it may be said that the 

 Lepidoptera is that in which the size and effectiveness of the pos- 

 terior wings are most nearly approximated to that of the anterior f . 

 The posterior wings of the Hymenoptera are decidedly inferior 

 to the anterior in size ; and it may perhaps be presumed that their 

 efficiency as organs of flight is subordinate to and dependent upon 

 the former, whose movements they are evidently formed to follow. 

 Lastly, in the Diptera, the posterior wings are only found under 

 the guise of halteres, and for purposes of flight are entirely ob- 

 solete. 



Thus in these three orders we are brought, by a succession of 

 stages, from a condition in which the size and effectiveness of the 

 wings are somewhat equal, to one in which the posterior are atro- 

 phied, and the power of flight is entirely concentrated in the me- 

 sothorax. Let us see if we can trace a similar succession in the 

 development of the segments themselves. If we can succeed in 

 showing that the comparative development of the two alary seg- 

 ments in the Lepidoptera and the Hymenoptera is in proportion 

 to their wing-power, as I may term it, we shall then have an 

 a priori ground for thinking that the comparative development of 

 the segments of the Diptera follows the same rule ; in fact, that 

 the metathorax is almost as obsolete as the wings, and that nearly 

 the whole of the thoracic region is mesothoracic. 



* Guide to the Study of Insects, p. 104. 



t From the absence of longitudinal dorsal muscles in the metathorax of the 

 Lepidoptera, I iucline to the opinion that even in this order the posterior wings 

 are subordinate to the anterior. 



