DIPTERA. —— ATHERICERA. 555 
generally completely withdrawn (when at rest) into the oral cavity ; 
occasionally (as in the Conopsidz ) it is porrected; but there are only 
two, or at most four (as in the Syrphide*), lancet-like organs. In the 
Bot flies (CEstride) the mouth is generally entirely rudimental. 
The larve are soft fleshy grubs, with the body very contracted, 
and ringed, without any appearance of legs; the front part is at- 
tenuated; the head is of a variable figure, and its external organs 
consist of one or two unguiform appendages, accompanied occasionally 
by fleshy lobes, and probably in all, by a kind of tongue destined to 
receive the nutritious fluids upon which it feeds. These larvae undergo 
the coarctate kind of metamorphosis, never shedding the skin in 
which they are enclosed at the period of their exclusion from the 
egg, and which hardens and becomes, when the larva has attained its 
full size, a kind of cocoon in which the pupa is enclosed; this is 
effected in the following manner: the larva by degrees contracts 
itself and becomes much shorter, assuming an oval form; the anterior 
part, which before was narrow, increases in thickness, and is sometimes 
even larger than the opposite extremity of the body; the segments 
become more and more indistinct until the insect appears under the 
form of an oval chestnut-coloured mass, in which scarcely any traces 
of rings or spiracles are visible. The body of the enclosed insect is 
detached by degrees from the inner skin of this cocoon or pupa- 
rium, as it may be considered, and appears under the figure of a very 
soft elongated mass, in which none of the parts of the future insect 
are at first visible; it soon, however, assumes the appearance of a 
pupa, in which the rudimental limbs are traceable. From this cocoon 
the perfect insect escapes by scaling off the interior part of the case, 
which it detaches by repeated efforts of the head, having also cast off 
the pellicle in which it was encased whilst a pupa. 
These insects in the perfect state are attached to flowers, leaves, 
&c., very few being carnivorous, or feeding upon other insects. 
This stirps comprises the Linnean genera Conops, Céstrus, and 
the greater portion of Musca; amongst the latter, a great number of 
species were placed by Linnzeus, the proboscis of which encloses four 
lancet-like organs (including the Syrphidz), instead of two, as in all 
the other Athericera, furnished with a proboscis. 
. 
* Hence Macquart, relying on the structure of the mouth, arranges this family 
amongst the tribes which have an incomplete instead of a coarctate pupa, 
