GNATS, MIDGES, AND MOSQUITOES 231 



or less use to society at large. It follows, then, that 

 their extermination from any district might not be 

 altogether an advantage, unless accompanied by other 

 changes, such as drainage, &c. ; and in estimating the 

 influence of mosquitoes, for example, in the economy of 

 nature, one has to set their services as scavengers over 

 against the annoyance they cause by sucking blood. It 

 might be a philosophical, if not very comforting, reflec- 

 tion for any one suffering from the persecutions of these 

 pests, that the more mosquitoes there are, the more 

 scavenging work must have been done in bringing them 

 to maturity, and the more must the sanitary condition 

 of the country round have been thereby improved ! 

 There is another curious fact connected with this 

 stage in the life-history of these insects : when fully 

 grown, as we have already seen, they subsist only 

 on liquid food, their mouth organs being excellently 

 fitted for taking in liquids, while they would find it 

 absolutely impossible to make any use of solid food. 

 But in this earlier stage, the conditions are reversed ; 

 solid food is the order of the day (though plentifully 

 steeped in water, it is true), and no sucking apparatus 

 exists, the mouth being armed with biting jaws instead. 



The change, however, is not suddenly made from the 

 one style to the other. There intervenes a condition in 

 which the insect takes no food at all, either solid or 

 liquid, having no available mouth ; for, when several 

 moultings of the jerky larva have taken place, it makes 

 another change of skin, which results in an entire 

 upsetting of all its arrangements. After this moult 

 it appears as a kind of animated " comma," with a big 

 head and a curved tail (Plate IV.). 



The apparent head is really head, thorax, beak, an- 

 tennse, limbs, and wings of the perfect insect, all bound 



