286 PTILOTA : IMAGO EXTERNAL ANATOMY. 



logous to the mesothoracic tegulac, or basal wing-covers, of 

 the Lepidoptera. He likewise considers them as somewhat 

 analogous to the lateral prolongation of the prothorax, ob- 

 served in some species of Psychodce and Scenopinus (dipte- 

 rous genera). As already observed, however, they have been 

 demonstrated to be the only mesothoracic appendages found 

 to exist in these insects (the wings being metathoracic) ; con- 

 sequently they must be regarded as analogues of the meso- 

 thoracic organs of flight. During the short time that these 

 insects live in the winged state, the pseudhalteres are ob- 

 served to quiver very intensely. 



2. Leys, or the Organs of terrestrial or aquatic Progres- 

 sion. If we have found the organs of flight constructed in a 

 manner totally unlike those of birds, we shall discover equal- 

 ly great differences in the construction of their legs. Thus, 

 whilst the former are provided with but a pair of organs for 

 progression on the land, winged insects have three'pairs, spi- 

 ders four pairs, crabs five pairs, and centipedes and millipedes 



more than r a hundred 



J33 -f3<> \^ legs. These organs 



arise from the ster- 

 num of the various 

 segments of the tho- 

 rax in the true insects, 

 and are employed not 



v d 



lT / . ,ft- ' ?**^ nin ^ creeping, and 

 climbing, but also in 

 running and swim- 

 ming, as well as in 

 various operations con- 

 nected with their eco- 

 nomv. They consist 



of a series of tubular joints, formed of a similar substance 

 with the remainder of the external parts of the body, and 



141 



