550 MODERN CLASSIFICATION OF INSECTS. 
longer than the head, with five distinct joints; the two terminal ones 
(which are distinct, although much smaller, in the Asilide and Em- 
pide) being large, and forming a club (fig. 129. 17. head and antenna 
of Mydas). The apical nerves run parallel with the posterior margin 
of the wing (fig. 129. 18.), as in Pangonia. The body is long, the 
legs strong, and the tarsi furnished with two pulvilli; the hind femora 
are thickened. 
These insects, according to Macquart, attack and devour other 
insects, seizing them with force whilst on the wing, and holding them 
with their fore legs.) Mr. MacLeay has informed me that the larvee 
of M. tricolor (which he observed in Cuba) are parasitic upon the 
larvee of the giant Prioni. In some of Latreille’s works we find this 
family united with the following; but the neuration of the wings is 
quite different. M. Macquart has entered into an inquiry as to their 
real affinities, and seems to have no hesitation in assigning to them a 
close relation with the Asilide. Such is also my opinion. 

The family THEerevip#, or the tribe Xylotome of Meigen, is of 
very small extent (jg. 129.19. Thereva plebeia), having the wings 
divaricate, and offering several perfect cells, formed nearly as in the 
Leptidz ; the apical nerves running to the extremity of the wing. 
The palpi are enclosed in the oral cavity. The proboscis is termi- 
nated in a pair of large membranous labial lobes, thus differing from 
the preceding Tanystoma. The third joint of the antennz is ovate 
conic, and terminated by a minute triarticulate style. The abdomen 
is conical. Many of the species are clothed with a silky kind of 
down. They are of moderate size, and are generally found upon 
trees; sometimes they are seen flying in swarms. The larva of 
Thereva plebeia, first observed by Frisch (Beschr. pt. 1. tab. 9.), and 
recently by Bouché (Naturgesch. tab. 4. fig. 16—20.), is very long, 
slender, and serpentiform (fig. 129. 20.) The head is small, with 
two short antennz. The body appears to consist of twenty distinct 
segments (at least the intermediate segments seem divided into two 
parts): the body is furnished at the extremity with two respiratory 
tubes. These larvee were found in vegetable garden mould. The 
pupe (fig. 129. 21.) are oblong, having the front of the head armed 
with two spines, and each side of the thorax with three, the middle 
one being the longest and curved. Meigen has also observed the 
