ORDERS OF INSECTS. ^57 



of fluid nourishment^ — they are hence called Haus- 

 TELLATA or Suckiug Insects. They form three prin- 

 cipal orders. 



The Lepidoptera, forming the first of these, are 

 characterized by the possession of a spiral trunk 

 formed by a modification of the maxillae; they are 

 usually furnished with four large membranous wings, 

 the surface of which is more or less clothed with 

 delicate scales. Their metamorphosis is complete. 



In the DiPTERA, the mouth consists of a short and 

 usually fleshy proboscis (the labium), enclosing from 

 one to six bristles, the representatives of the other 

 parts of the mouth ; these insects are readily distin- 

 guished by the possession of only a single pair of 

 wings, behind the insertion of which we find a pair of 

 knobbed filaments (halteres) which are considered to 

 represent the hinder wings. Their metamorphosis is 

 also complete. 



Lastly, the series of Metabolous Insects is closed by 

 the Rhynchota, in which the labium forms a jointed 

 rostrum, enclosing four bristles representing the man- 

 dibles and maxillae ; the anterior wings are usually of 

 a firmer texture than the hinder ones, which in most 

 cases are folded up beneath them. The metamor- 

 phosis of the Rhynchota is incomplete. 



In addition to these, the Haustellate section of In- 

 sects includes a fourth small order, that of the Apha- 

 NiPTERA, including only the different kinds of Fleas ; 

 this group may be arranged close to the Diptera. 



The Ametabolous insects are also divided into three 

 orders : — of these, the Anoplura, or Lice, have a suc- 

 torial mouth; the Mallophaga, or Bird-lice, have a 

 biting mouth, and the extremity of the abdomen 

 destitute of any appendages ; whilst the Thysanura, 



D 5 



