MR. LUBBOCK ON TWO AQUATIC HYMENOPTERA. 141 
forms a club-like mass, which appears to be composed of three segments; the last joint, 
however, is very indistinctly marked. 
The legs are much like those of the preceding species; but the fourth segment of the 
tarsus (Pl. XXIII. fig. 15) is, in all the legs, very small. Mr. Westwood*, indeed, describes 
the tarsus of his species as being only three-jointed, and in this he is confirmed by 
Mr. Haliday T. Mr. Westwood's figure, however (fig. 9), shows a fourth segment, which, 
though small, is even larger in proportion than that of my species. I am disposed, 
therefore, to consider that the tarsus of Trichogramma is in reality four-jointed, and 
that Westwood's and Haliday's descriptions must be amended. 
The legs are not compressed, nor have they any fringe of hairs. They show, therefore, 
no trace of adaptation to their new function. 
The anterior wings (Pl. XXIII. fig. 13) are club-shaped, narrow at the base, expanding 
almost to the apex, with elegantly curved margins. The single nervure extends for 
about half the length of the wing, and at the apical extremity turns inwards, and ends 
abruptly. The margin of the wing is fringed by long setze, which are largest along the 
club-shaped extremity of the wing, from which they gradually decrease in length in each 
direction. The insertion of the setze is somewhat peculiar (Pl. XXIII. fig.13 a), and makes 
a pretty border to the wing. Near and parallel to the front and hinder margins are two 
rows of small spines, and others are scattered about the expanded portion. 
The hind wings are narrow, linear, ciliated, situated on a long pedestal, and, in fact, 
so closely resemble those of Polynema that they need not be separately described. 
Remarkable and peculiar as is undoubtedly this conformation of the wings, still, as 
already observed, it is the same as that which occurs in many allied species, and is not 
therefore to be regarded as in any way connected with the aquatic habits. 
Moreover, we must remember that though the wings of Polynema natans so closely 
resemble those of Prestwichia, and though both insects are essentially aquatic in their 
habits, yet the one swims only with its wings, the other with its legs. 
While the abdomen of Polynema is petiolated, that of Prestwichia is sessile, and tapers 
almost from its base to the insertion of the ovipositor; so that the whole body is almost 
boat-shaped. This latter organ is about as long as the abdomen, very stout in propor- 
tion, and bears a few scattered hairs. 
My specimens were found in the same pond as P. natans, from the beginning of August 
to the middle of September. It appears, however, to be rarer; and I only found six 
specimens, all of which were females. Possibly the male is not aquatic in his habits. 
* Lond. and Edin. Phil. Mag. 1833, p. 444. + Ent. Mag. vol. i. p. 341. 
VOL, XXIV. U 
