STATES OF INSECTS. (Larva.) 125 
found only in those larvee which have the power of spin- 
ning silk; that is, in all Lepidoptera, most Hymenoptera, 
Trichoptera, some Neuroptera, and even a Dipterous in- 
sect?. This tube, Lyonnet had reason to believe, is com- 
posed of longitudinal slips, alternately corneous and mem- 
branous, so as to give the insect the power of contracting 
its diameter, and thus making the thread thicker or 
smaller. There is only a single orifice at the end, which 
is cut obliquely, somewhat like a pen, only with less obli- 
quity, and without a point, the opening being below, so 
as to be conveniently applicable to the bodies on which 
the larva is placed. Reaumur conceived that this spin- 
neret had two orifices; but Lyonnet ascertained this to 
be a mistake, the two silk tubes uniting into one before 
they reach the orifice. From the contractile nature of 
the sides and the form of the orifice, combined with the 
power the insect has of moving it in every direction, re- 
sults the great difference which we see in the breadth and 
form of the threads, some being seven or eight times as 
thick as others, some cylindrical, others flat, others chan- 
nelled, and others of different thickness in different parts. 
In the larvae of many Diptera the under-lip is merely a 
small tubercle, which can be protruded from the insect’s 
mouth by pressure ©. 
One of the most remarkable prehensile instruments, in 
which the art and skill of a Divine MEcuHanician are 
singularly conspicuous, and which appears to be without 
a parallel in the insect world, may be seen in the under- 
lip of the various species of dragon-fly (Zzbellulina). In 
* De Geer vi. 370. This species (T%pula Agarici seticornis De Geer) 
has two separate spinnerets. ¢. xx. f. 8. m m. 
* Lyonnet 55—. © Reaum. iv. 166, 
