2/2 PTILOTA : IMAGO — EXTERNAL ANATOMY. 



fig. 124, o), which sometimes, as in some of the Procto- 

 trupidce (especially Galesus,) are of a large size, must be no- 

 ticed. They also acquire a large size in the Lepidoptera. 



The mesothoracic appendages are the anterior pair of 

 wings, the middle pair of legs, and the mesothoracic spiracles. 

 In dipterous insects the base of the wings is often furnished 

 with a membranous scale, termed the alula or winglet, which 

 Kirby and Spence regard, but incorrectly, as the true ana- 

 logue of the second pair of wings, and consequently as at- 

 tached to the metathorax (see fig. 126 y). 



3. The Metathorax, T 3 upper, and (T) 3 under siu-face, 

 {Protrunk, Kirby and Spence). — This is the last of the three 

 thoracic segments (or the second of the thorax of Strauss, or 

 alitrunk of Kirby and Spence), and bears the posterior pair 

 of wings, or their representatives, and the third pair of legs. 

 Its developement and consistence are variable, dependent upon 

 the principles abeady stated to regulate the developement of 

 the mesothorax. It consists of the same number of pieces 

 as the preceding segment, the dorsal pieces (composing the 

 metanotum), being the prsescutum (mesophragma of Kirby 

 and Spence in Coleopteraj postdorsulum of the same authors 

 in Hymenoptera), scutum (postdorsulum of Kirby in Coleo- 

 ptera,) scutellum (postscutellum and postfrsenum of Kirby), 

 and postscutellum (metaphragma, Kirby and Spence). The 

 scutum and scutellum are the most conspicuous parts in 

 Coleoptera, being channelled longitudinally. In Hymenoptera, 

 (see fig. 124, T 3), the metathorax is reduced in size, and this 

 is more especially the case in Diptera, (see fig. 126, T 3) ; but 

 in the parasitic Strepsiptera, the fore-wings are very minute 

 and twisted appendages, and the second pair of wings of 

 very large size, the metathorax being so greatly increased in 

 its dimensions that it may be almost said to constitute the 

 entire thorax. 



The under surface of the metathorax or the metasternum 



