THE EXTERNAL ANATOMY OF INSECTS 55 



The first axillary. This sclerite (Fig. 66, i Ax) articulates with 

 the anterior notal wing-process and is specially connected with the 

 base of the subcostal vein of the wing. In rare cases it is divided 

 into two. 



The second axillary. The second axillary (Fig. 66, 2 Ax) articulates 

 with the first axillary proximally and usually with the base of the 

 radius distally; it also articulates below with the wing-process of the 

 pleurum, constituting thus a sort of pivotal element. 



The third axillary. The third axillary (Fig. 66, 3 Ax) is interposed 

 between the bases of the anal veins and the fourth axillary when this 

 sclerite is present. When the fourth axillary is absent, as it is in 



Fig. 66. Diagram of a generalized wing and its articular sclerites (From 

 Snodgrass). 



nearly all insects except Orthoptera and Hymenoptera, the third 

 axillary articulates directly with the posterior notal wing-process. 



The fourth axillary. When this sclerite is present it articulates 

 with the posterior notal wing -process proximally and with the third 

 axillary distally (Fig. 66, 4 Ax). Usually this sclerite is absent; it 

 occurs principally in Orthoptera and Hymenoptera. 



The median plates. The median plates of the -wing-joint are not 

 of constant shape and occurrence; when present, these plates are 

 associated with the bases of the media, the cubitus, and the first anal 

 vein when the latter is separated from the other anals. Often one of 

 them is fused with the third axillary and sometimes none of them are 

 present. 



d. THE APPENDAGES OF THE THORAX 



The appendages of the thorax are the organs of locomotion. 

 They consist of the legs and the wings. Of the former there are three 



