STATES OF INSECTS. 125 



found only in those larvae which have the power of spin- 

 ning silk ; that is, in all Lepidopte?-a, most Hijmenoptera, 

 Trichopiera, some Neuroptera, and even a Dipterous in- 

 sect a . 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 b . 

 In the larvse of many Diptera the under-lip is merely a 

 small tubercle, which can be protruded from the insect's 

 mouth by pressure c . 



One of the most remarkable prepensile instruments, in 

 which the art and skill of a Divine Mechanician 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 (Libellula L.). In 



a De Geer vi. 3/0. This species (Tipula Agarici seticornis De Geer) 

 lias two separate spinnerets, t. xx./. 8. m m. 



b Lyonnet 55 — . e Reaum. iv. IGG. 



