STRUCTURE OF THE DIPTERA. 357 



host of mse^ ts, the most general character of which is 

 to possess two membranous wings, with their disk 

 variously occupied witli longitudinal uervures and com- 

 paratively few transverse ones, these wings are affixed 

 to the mesothorax, and are thus analogous to the 

 superior wings of those insects which have four genuine 

 wings, or to the elytra or hemelytra of those in which 

 the superior wings are supplanted by such appendages. 

 In addition to these two wings, and attached behind 

 them, are a couple of usually clavated organs, having a 

 moderate peduncle ; these are called halteres, or poisers ; 

 they are articulated organs, but the uses of which are 

 not known, although they may not be considered as 

 analogous to the secondary wings of the other orders. 

 In addition to these organs, very many possess a con- 

 cavo-convex scale, which covers the halteres, and v/hich 

 is called the alula, or winglet. Being exclusively suc- 

 torial insects, and feeding upon liquids, their trophi, or 

 organs whereby they take their food, consist of a flexible 

 proboscis, modified in a variety of ways in the different 

 families, but always analogous to the structure we have 

 before described in mandibulated insects : thus, we find 

 the theca, or sheath, which represents the labium, and 

 which encases the proboscis ; further, the haustellum, 

 which consists most generally of two pieces analogous 

 to the labrum ; occasionally of two others, which are 

 called the cultelli, or knives, which possibly represent 

 the mandibles ; and there are rarely two others, called 

 the scalpella, which may remind us of the maxillae ; 

 and there are, in addition to these, two palpi. It thus 

 amply differs from the trophi of all other haustellate 

 insects. These are the most general characters, whereby 

 the entire order is distinguished. Within themselves, 

 the modifications of structure which they exhibit, furnish 

 the characters upon which their distribution is founded. 

 The most prominent of these are supplied by their 

 antenna?, which, considered typically, consist of but 

 three joints ; but, attached to the third, we frequently 

 detect a setaceous appcnda e. In the first division. 



