Descriptions of British Diptera. 



147 



the two lower 

 joints subcylin- 

 dric, the third 

 somewhat thick- 

 ened, obovate and 

 pilose ; (Fig. 2, 

 a.) thorax oval, 

 pubescent ; scu- 

 tellum semicir- 

 cular, armed pos- 

 teriorly with two 

 spines; abdomen oval, flat or very slightly convex ; wingletsandhal- 

 teres rather small ; neuration of the wings as in the accompanying 

 figure (3.) 



Larva aquatic, breathing by means of a star-like radiated appara- 

 tus attached to the tail. 



Stratiomys is placed at the head of this family, as exemplifying 

 in the most characteristic manner the peculiarities by which it is dis- 

 tinguished. As above defined, this genus comprehends six British 

 species, nearly all that have been ascertained to inhabit Europe. 

 They are all of pretty large size, and very similar to each other in 

 colour, and the distribution of their markings. The larvae, which 

 are admirably delineated and described by Swammerdam, under the 

 name of Asilus,* are of singular and elegant structure, especially in 

 their respiratory organs. In the shape of the body they are some- 

 what fusiform, thickest before the middle and tapering to both ends, 

 the caudal segment much more elongated than the others, and sup- 

 porting at the extremity a circle of about thirty diverging rays. 

 These rays are beautifully feathered, capable of repelling water, and 

 placed round an orifice in the tail, communicating with two very 

 large tracheae or air-vessels. When the insect desires a supply of 

 fresh air, the tail is raised to the surface, and these rays expanded 

 on all sides ; a free communication is thus opened by a kind of ob- 

 conical funnel, between the atmosphere and the aperture in the tail, 

 even though the latter be a little beneath the level of the surface. 

 When about to descend, the points of the hairs are drawn towards 

 each other, and a globule of air enclosed, which shines in its pas- 

 sage through the water like a drop of quicksilver. The use of this 

 air-bubble seems to be both to afford a continued supply of air, and 

 to render the body more buoyant when it becomes necessary to re- 

 ascend. To facilitate the same object when the air happens to be 

 exhausted while beneath the surface, the animal seems to possess 



* Book of Nature, (Hill's edition,) p. 43-62 ; pis. 39-42. 



