With the exception of Hydrous piceus , the true Dytici are 
the largest insects that inhabit the waters of Europe, and until 
lately there were but 3 species recorded as British. As their 
habits and economy are similar to the Acilii s I shall prefer 
giving the specific characters, and must refer to the detailed 
accounts of De Geer, Roesel, and Latreille, for their histories. 
I. Furcate process of sternum obtuse. Thorax not en- 
tirely margined with yellow. 
1. D. punctulatus Fab. Ent. Syst. v. 1. pars 1. 188. 4. Don. 
Brit. Ins. 15. 540. Panz. F. G. 110. 13 & 14. 
Much the smallest; sides only of thorax yellow. 
Antennae robust, articulations short; each elytron 
of female with 9 striae. Sternum very obtuse (fig. P). 
2. dimidiatus Gijll. Ins. Suec. v. 1. p. 469. n. 4. 
Much the largest, sides of thorax yellow, and a nar- 
row ferruginous line at the other margins, 3 basal 
joints of antennae much more robustthan the others, 
each elytron of the female with 10 striae, extending 
scarcely beyond the middle (D). 
II. Furcate process of sternum acute. Thorax com- 
pletely margined with yellow. 
a. Anterior claws large in the males. 
3. marginalis Linn. Syst. Nat. 2. 665. 7. Don. Brit. Ins. 5. 
161. Panz. F. G. 86. 3 & 4. 
Spines of sternum rather short, not very acute (M). 
4. circumflexus Fab. Syst. Eleut . 1. 258. 4. 
Spines of sternum slender, elongated, very acute (C). 
b. Anterior claws small in both sexes. 
5. angustatus Stephens’s MSS. Nobis. 
Spines of sternum diverging, considerably acute (A). 
6. flavo-maculatus Lat. Hist. Nat. ? v. 8. p. 162. n. 3. 
Elytra smooth in both sexes ? Sternum like the last ( F). 
This last is a most extraordinary anomaly, respecting which 
the most learned entomologists are completely at variance. 
My specimen proving upon dissection to be a female, as well 
as 3 others dissected by Professor Bonelli, I cannot but con- 
sider it a variety of the female of D. angustatus , with which 
sex it perfectly agrees in structure, although in sculpture it 
has every appearance of the males of that species. 
All the Dytici are found in ponds and ditches during May, 
June, and July. The fine species figured, as well as D. an- 
gustatus , were first discovered at Whittlesea Meer by Mr. Chant 
and Mr. Bentley, to whom I am indebted for examples of the 
different sternums by which the species are so easily distin- 
guished ; and Dr. Stephenson has since captured them in the 
same neighbourhood, and has obligingly favoured me with 
specimens. 
