98 ENTOMOLOGICAL NEWS. [March, '09 



have been due to great conceptions of generalization, and his- 

 tory furnishes many examples where such ideas have arisen 

 from inaccurate observations or have caused their author to 

 see the facts in a distorted manner. So Audouin did a great 

 thing for entomology when he published, in 1824, his generali- 

 zations on the insect thorax, though in establishing his theory 

 he erroneously described the facts in some cases. It is really 

 fortunate that no entomologist at that time made a critical ex- 

 amination of his data for the whole idea of reduplication of 

 parts in the successive segments might have been then discredit- 

 ed. Yet it is remarkable that no one at the present time has 

 pointed out how palpably inaccurate are Audouin's drawings 

 of the meso and metatergum of Dytiscus circninflcxus Fab. on 

 which he bases his generalizations on the tergal structure. 



Figure I, Plate VI., is a drawing of the metatergum of D. 

 circumflexus. On the right are shown the axillary sclerites 

 (i Ax, 2 Ax, 3 Ax} by means of which the wing is articulated 

 to the metathorax. The dotted parts represent membrane. 

 Now, it is perfectly clear that this tergum consists of only two 

 actually separate plates, but these two are clearly and distinct- 

 ly separated by a definite suture which is widely membranous 

 toward the sides and narrowly so in the middle. The anterior 

 plate bears the wings entirely, its posterior angles being pro- 

 duced into the axillary cords (AxC) or corrugated cord-like 

 thickenings of the anal margins of the basal or axillary mem- 

 branes of the wings. The second or posterior plate (PN) is 

 comparatively narrow. It is entirely free from the wings, 

 bears the postphragma (Pph} of the metathorax on its pos- 

 terior margin, and laterally articulates with the epimera of the 

 metathorax by special processes (i). 



The metatergum of Dytiscus is, thus, composed actually of 

 only two plates one bearing the wings, the other free from 

 the wings and connected with the epimera behind the wing 

 bases. This structure is, moreover, characteristic of the meta- 

 terga of all adult Coleoptera and Euplexoptera and of both the 

 meso and metaterga of all the other insect orders except the 

 Aptera and the Orthoptera. The second plate is lacking in all 

 nymphal forms and in the pupae of the lower Holometamor- 



