xxii EXTERNAL STRUCTURE 493 



perfect Insect varies greatly. In Tenthredinidae nine can be 

 distinguished, while in some of the Chrysididae it is difficult to 

 detect more than three behind the petiole. These distinctions 

 are, however, superficial or secondary, being due to changes in 

 the later life in connexion with the stings or borers ; in the 

 larvae that have been examined thirteen segments behind the 

 head have usually been detected. 



Nothing is more remarkable in the Hymenoptera than the 

 great differences that exist in the form of the petiole. This may 

 be very short, as in the bees, so that the abdomen when not 

 deflexed does not appear to be separated from the thorax (Fig. 

 331, C); in this condition it is said to be sessile, a term which 

 it would be well to replace by that of pseudosessile. In many 

 of the solitary wasps the petiole is very long. In ants it is re- 

 placed by one or two curiously-shaped small segments called 

 nodes (Fig. 60, B, 2, 3), and in many ants these are provided 

 with structures for the production of sound. The abdomen is 

 formed by a system of double imbrications ; each dorsal plate 

 overlaps each ventral plate, and the hind margin of each segment 

 embraces the front part of the one following ; thus this part of 

 the body has not only great mobility, but is also capable of much 

 distension and extension. The pleura are apparently absent, but 

 each one has really become a part of the dorsal plate of the seg- 

 ment to which it belongs. This is shown to be the case by 

 Cimbex, where the division between pleuron and dorsal plate 

 exists on each segment except the basal one. Owing to this 

 arrangement, the abdominal stigmata in Hymenoptera appear 

 to be placed on the dorsal plates. 



The organs for mechanical purposes existing at the extremity 

 of the body in Hymenoptera exhibit a great diversity of form ; 

 they are saws, borers, piercers, or stings. Notwithstanding 

 their great differences they are all, in their origin, essentially 

 similar, and consist of six parts developed from limb-like pro- 

 longations on the penultimate and antepenultimate segments of 

 the larva, as described by Packard and Dewitz. 1 These processes 

 have by some been thought to be not essentially different from 

 abdominal legs, and Cholodkovsky has recently advocated this 



opinion. 2 



1 Zeitschr. wiss. Zool. xxv. 1874, p. 184. 



2 Ann. Jfoij. yt. Hist. (6) x. 1892, p. 442. 



