CLASS INSECTS. 



449 



example, which is always apterous, like the bug, and which 

 has been considered by most naturalists entitled to form a 

 particular order, called suckers. 



545. The order of diptera is characterized by the exist- 

 ence of a single pair of membranous wings, not unlike those 

 of the hymenoptera, and by the structure of the mouth, 



Fig. 479. Pyrales of the Vine; Caterpillar of the Vine Moth.* 



arranged for suction only ; we observe in it in general a pro- 

 boscis, sometimes horny and elongated, sometimes soft and 

 retractile, and inclosing stiff and sharp bristles. 



A sufficiently correct idea of the general form of dipterous 

 insects may be derived from one of them known to all the 

 world the common fly; and we shall only add, that all un- 

 dergo a complete metamorphosis. The larvsB (Fig. 486, a) 

 have no limbs ; their head is soft, and the mouth is generally 

 provided with hooks. Sometimes they change the skiii 

 several times, and spin a cocoon, to transform themselves in 

 it into a nymph j at other times they do not moult, and their 



* Leaf of the vine attacked by the pyrales : 4, the male ; 4a, the female 

 46, the caterpillar ; 4>c, the eggs ; 4d and 4<e, the chrysalides. 

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