702 EXTERNAL ANATOMY OF INSECTS. 
cases however the reverse takes place. In the sexes also 
there is frequently a difference in the number of seg- 
ments, as has been before observed *. I shall express 
the variations in question by two figures, the first repre- 
senting the number of dorsal segments, and the second 
the ventval—they usually only express the apparent seg- 
ments: perhaps a very general examination and dissec- 
tion might bring many of them nearer to a common type. 
1:1. Chelonus. 7:6. Dytiscus 2. 
3:3. Chryside”. 7:7. Ammophila, &c. 
4:2. Leucospis °. as6 Dytiscus 3. 
5:5. Syrphus. Lucanus, &c. 
5:6. Halictus ?. : 7. Geotrupes. 
6:5. Nepa. :8. Pimpla. 
8 
8 
ei Halictus &. §: 10. Euchlora? 
Belostoma. 8:13. Cermatia. 
: Curculio L. 9:5. Carabus. 
9 
ieee 
Cerambyx L. :6. Gymnopleurus. 
@ See above, p. 338. 
> There is reason to suppose that in Chelonus and the Chryside 
several segments are retracted within the abdomen, and if the cavity 
of its under side in the latter be examined, it will be discovered that 
the epigastrium is divided in the middle into two pieces, and that both 
the sides of this and the following segments are covered by three 
narrow accessory plates, one to each, the last being the shortest. 
De Geer (ii. 833.) describes Chrysis ignita as having four abdominal 
rings; but this is only in appearance, there being really only three. 
This appearance is produced by the apex of the last dorsal segment 
being more depressed and marked with several deep little excava- 
tions that look like holes. In some species of Sti/bum this segment 
consists as it were of three ridges or steps. 
° In this genus the ventral segments are replaced by a long narrow 
central plate, succeeded by a minute one. 
4 In this genus the bed of the posterior coxe appears to consist 
of two segments, which are beautifully fringed with parallel short 
bristles. 
