TYPE TRACHEATA. 



489 



or two unsegineiited plates of which the innermost is usually 

 toothed. The second maxillae (Fig. 225, A) are also jointed 

 and are fused together to form a Iow 7 er lip, or labium. The 

 fused basal joints form the 

 submenlum (smj, the second \1 

 joints the mentum (m], which \ 

 bears, as does the stipes of the 

 maxillae, a jointed palp (p) and 

 terminates frequently in two 

 uujointed plate-like processes. 

 The three pairs of appendages 

 of the thorax are typically 

 ambulatory, but are modified 

 for clasping, swimming, digging, 

 etc., according to the habits of 

 the insect. They typically con- 

 sist of a basal joint, the coxa, 

 succeeded by one or two small 

 joints, the trockanter, upon 

 which follow a femur, a tibia, 

 and a tarsus, the last consisting 



of live (occasionally four) short joints, the terminal one bear- 

 ing two claws or ungues. The abdomen in the adult forms is 

 as a rule, destitute of appendages, except in the Thysanuraus 

 (Fig. 231), the lowest of all the orders of Insects. In these a 

 number of the segments are provided with a pair of spurlike 

 processes which recall the spurs upon the basal joints of the 

 trunk appendages of Scolopendrella, and are apparently ho- 

 mologous with them. In the embryos of probably all forms 

 rudimentary appendages are found on some of the abdominal 

 segments, but they later disappear, showing, however, a de- 

 scent of the Insecta from forms in which abdominal appen- 

 dages were functional in the adult. Processes of various 

 kinds, such as cerci, ovipositors, and copulatory organs, are 

 frequently borne by the posterior abdominal segments, but 

 these do not seem to be equivalent to appendages. 



As stated, a pair of wings is usually borne by the meso- 

 and metathoracic segments. These structures are entirely 

 wanting in the lowest insects, the Thysanura and Collembola, 



B 



FIG. 225. MOUTH-PARTS OF A BEE- 

 TLE, Monohammus, 

 A = labium. 

 B = maxilla. 

 G = mandible. 



