DIPTERA. 501 
transverse diameter, and is composed of from five to nine joints, ter- 
minated in a point in the females ; in some of the latter the number of 
joints is reduced; the apparently wanting segments are, however, con: 
verted into a telescopic-formed ovipositor. 
The transformations of this order are either of the incomplete or 
coarctate kind, or rather such is the nature of the pupa state. 
The larve are fleshy, cylindric, footless grubs* ; but some species 
are furnished with representatives of legs. It is in this order alone 
that the head is found to exhibit a soft, fleshy, and variable structure ; 
a character, however, exclusive to those Diptera which have a coarctate 
pupa. The mouth is generally furnished with two hooks, which are 
thrust into the substances from which the larve derive their nutriment. 
In the majority of these larvee the spiracles, two in number, are situated 
at the posterior extremity of the body; many are also furnished with 
a pair of these organs on the segment immediately succeeding the head, 
and in some they are placed on several of the consecutive segments. 
When arrived at their full size, the larvee of some of the species 
(Nemocera and Tanystoma) cast their skin, and appear in the form 
of incomplete pupz, having their limbs enclosed in distinct sheaths. 
The pupz of the Culicidz are not, however, quiescent. ‘This trans- 
formation is sometimes effected in a cocoon woven by the larva. In 
the majority of the order, however (Muscide, &c.), the outer skin is 
not shed, but, by degrees, contracts and hardens, until it assumes 
the appearance of an oval brownish shell or case, within which the body 
of the Jarva is detached in a soft and gelatinous mass, and which ex- 
hibits no appearance of limbs or joints; by degrees, however, these 
parts are found (on opening the shell) to have become distinct when 
the insect is in the true state of a coarctate pupa. In some species, 
however, the larva skin scarcely changes its form, on the insect’s as- 
suming the pupa state. The perfect insect makes its escape trom this 
case by causing the upper extremity to scale off, having also sloughed 
off the real envelope of the pupa. In the forest flies, Hippoboscide, a 
remarkable variation occurs; the insect passing the larva state, and 
* Bouché (in Nova Acta Natur. Curios. vol. xvii.), MacLeay (in Zool. Journ. 
vol. ii. No. 5., and Taylor’s Philos. Magazine, 1827; Bull. Féruss. February 1829), 
and Dufour, in a memoir presented to the “ Institut,” upon some fungivorous Dip- 
terous larve presented to the “ Institut,” and noticed in the Révue Zool. Soc. Cuvier. 
1839, No. 7., haye entered into numerous general details relative to the larve of 
this order. 
Keke 
