1864.] 559 



which have a quiescent pupa, it is undoubtedly the general rule, that 

 they have only one pair of thoracic spiracles, vphich is situated on the 

 prothorax. or immediately behind it. or sometimes on the anterior part 

 of the mesothorax (Elateru/x.) But still there are plenty of them 

 which have both meso- and meta-thoracie spiracles. As I purpose en- 

 tering fully on this and certain allied subjects in a future Paper, it will 

 be sufficient to refer here, in confirmation of this last point, to Westw. 

 Introd. I. p. 67, fig. 8, and compare p. 68; p. 255. II. p. 2o9. fig. 5; p. 

 252 ; p. 263, fig. 9 ; p. 267, fig. 15. 



Osten Sacken has said, that " the use and homology of the breast-bone 

 is unknown," and suggests that it may possibly represent the mentum 

 of the larva of Tipularise. {Dipt. N. A. p. 182.) Say, from his de- 

 scription of this part in the larva of Ore. destructor, appears to have 

 considered it as a pair of rudimentary legs, which it can scarcely be, 

 because it is one solid piece ; and besides, there is no instance in Insecta 

 of the development of only a single pair, or of only two pair of legs, 

 though in the larva of Passalus (Coleoptera) the hind pair of legs are 

 greatly reduced in size, and functionally impotent,* and in the imagos of 

 many Butterflies the same thing occurs in the front legs. (»S'fl_y'.s Works, 

 II. p. 5.) From the fact that in many species, especially those where 

 it assumes a Y-.shaped form, it is manifestly overlaid by the tran.sparent 

 integument of the insect, as may be seen from viewing it in different 

 lights, I infer that it is not any part of the external skeleton, and can- 

 not, therefore, be homologous with the central piece of the sternum in 

 the imago, or the mentum in the larva of Tipularise; and that it must 

 consequently be the homologue of some internal organ, perhaps the 

 " antecoxal plates" of Coleoptera. (Lee. Intr. Col. p. xv.) From the 

 fact stated by Osten Sacken, and which I can confirm from my own 

 observation, that this organ is peculiar to the larva of Cecidomyidie, 

 and from the further facts that its anterior extremity, as stated by the 

 same author, either bears one or two thorns or is serrated, &c., {Dipt. 

 N. A. p. 182,) and that when the head is retracted, as is usual in the 



*I state this of my own knowledge of P. cornutus Fabr. A larva o{ Passalus 

 was represented with only four legs by Abbot, apparently from overlooking the 

 hind legs, which are decussated on the sternum and not very obvious. (See 

 Westw. I7itr. I. p. 189.) 



