EXTERNAL ANATOMY OF INSECTS. 703 



insects; I have noticed more than twenty such variations, 

 and probably there are many more. Before I give you 

 them in detail, I must first observe that the dorsal and 

 ventral segments, though sometimes they correspond in 

 number, yet very often do not, the dorsal most com- 

 monly exceeding the ventral by a segment; in a few 

 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 a . I shall express 

 the variations in question by two figures, the Jirst repre- 

 senting the number of dorsal segments, and the second 

 the ventral — 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. 6:5. Nepa. 



3:3. Chry sides b . k-k S Halictus $ . 



4:2. Leucospis . (Belostoma. 



5 : 5. Syrphus. ^ _ _ f Curculio L. 



5 : 6. Halictus ? . \ Cerambyx L. 



a See above, p. 339. 



b There is reason to suppose that in Chelonus and the Chrysidce 

 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 

 beino- more depressed and marked with several deep little excava- 

 tions that look like holes. In some species of Stilbum this segment 

 consists as it were of three ridges or steps. 



c In this genus the ventral segments are replaced by a long narrow 

 central plate, succeeded by a minute one. 



