THE EXO-SKELETON OF THE ABDOMEN. 209 



The small sclerites of the wings have been described by Jurine in the 

 Hymenoptera [71], by Chabrier in various insects [72], and by Straus 

 Durckheim [40] in the cockchafer. The descriptions are not in all cases 

 clear, and it is difficult always to recognise the parts indicated. The 

 following are the most important synonyms : 



Parapteron, Audouin. Claviculaire, Chabrier. Petit radial, Jurine [72, 



vol. viii., p. 73]. 



Pre-epaulet, Straus Durckheim. 

 Epaulet, Straus Durckheim. 



Coracoid, Mihi. Bee de 1'humdrus, Chabrier [72, vi., pi. iS]. 

 Unguiculus, Mihi. Base de Fhumdrus, Chabrier in cockchafer. Ongulaire, 

 Chabrier in Hymenoptera. Petit cubital, Jurine [72, vol. viii., p. 73]. 

 Dens, Mihi. Omoplate and sigmoidea united, Chabrier. Grand and 

 petit humeral, Jurine. The sclerite called sigmoidea only occurs, 

 according to Chabrier, in the Hymenoptera. It is evidently a part of 

 the dens [72, vol. viii., p. 73]. 



Remigium, Mihi. Tige basilaire de 1'hume'rus, Chabrier. Chabrier 

 figures the hypopterygium of the cockchafer, but does not mention it 

 in the text [72, vol. vi., pi. 18]. 



5. THE EXO-SKELETON OF THE ABDOMEN, 

 a. General Morphology. 



The exo-skeleton of the abdomen is as remarkable for its 

 great simplicity and similarity of structure in all insects, as that 

 of the thorax is for its complexity and diversity. 



The abdominal skeleton consists of a series of annuli, of 

 which the typical number is nine, except in the Orthoptera, 

 where there are eleven. The ninth is frequently either rudi- 

 mentary or absent in the imago, its place being taken by a 

 small triangular dorsal plate, very similar to the telson of a 

 Crustacean. 



Several, commonly the three last, segments are invaginated 

 and withdrawn within the segment in front of them, and form 

 a cloacal pouch, or a tubular ovipositor. This arrangement 

 reduces the visible external segments in many of the Diptera to 

 five, and the first of these is not always apparent without 

 dissection. 



Each annulus of the abdominal skeleton consists of a dorsal 

 arch and a ventral plate. 



True paired appendages arise from the ventral plate of the 



