EXTERNAL ANATOMY OF INSECTS. 409 



external appearance being confined to a small triangular plate 

 which has its base situated against the hinder margin of the 

 prothorax, and its sides against the osseous proalce : in all 

 descriptions of Coleoptera the mesothorax is termed scidellwn. 

 In Orthoptera, with the exception of the Mantiies, the meso- 

 thorax has no external development when the wings are 

 closed: it is here at its m'mimum. In Hemiptera it reappears 

 as a triangular plate, similar to that called scutellum in Cole" 

 optera; as we proceed through the class we find it increases 

 in volume until, in Cicada, it has reached a magnitude 

 and prominence closely resembling its appearance in JLepi- 

 doptera. 



The fourth segment is the metathorax ; it bears the 

 metalce, or hind-wings. In Lepidoptera the expansion of the 

 hind-wings is considerable ; and accordingly this segment is 

 also very apparent. In Diptera the hind wings do not answer 

 the purpose of flight, but have dwindled to mere pedunculated 

 knobs, called halteres, or poisers ; the metathorax, conse- 

 quently, is very much diminished, and scarcely visible, being 

 nearly concealed, especially in the Muscce, by the enlargement 

 of a portion of the mesothorax. In Hijmenoptera it reappears 

 conspicuously, and increases in volume as the hind-wings 

 increase in expansion. In a small natural order of insects 

 which are parasitical on bees (Genus Stylops), the metathorax 

 is said to be very fully developed; but I have never yet 

 possessed the means of examining one of these insects under a 

 sufficiently high power to obtain any correct idea of its struc- 

 ture. From the elegant illustrations of Elenchus Walkeri, 

 (Curtis,) and Halictophagus Curtisii, (Curtis,) it would appear 

 that the epiphysal appendages which have been supposed 

 analogous to \he proalce of Coleoptera, arise from the ^jro- 

 thorax close to the insertion of the propedes ; the same may be 

 inferred from Kirby's definition, " Pseudelytera twisted attached 

 to the fore-leg." Now, if this be the case, these appendages 

 are not analogous to the proalce, but dXQ processes of the 2irotho- 

 rax.^ We see in the Coleoptera the cause of metathorac'ic de- 

 velopment, viz. the bearing of the sole organs of flight ; but in 

 Stylops, &c. we find the wings on the next segment to that which 



* " J'ai considere ces organes comme des pieces analogues aux pt^rygodes 

 <des Lepidopteres." — Latreille. Cours d'Entamolpgie. Tom. I. p. 242. 

 NO. IV. VOL. I. 3 G 



