xxii EXTERNAL STRUCTURE 489 



dimorphic form. In the parasitic division there are species that 

 are apterous in both sexes. The structure of the outer skeleton, 

 or external part of the body, exhibits some peculiarities, the chief 

 of which is the detachment of the side-pieces of the prothorax and 

 their great development. Not less remarkable is the abstraction 

 of a segment from the abdomen to become, as it were, part of 

 the thorax ; while between the first and second true segments of 

 the abdomen there exists a joint, or articulation, of the utmost 

 mechanical perfection, enabling the operations of stinging and 

 piercing to be executed with an accuracy that cannot be surpassed. 

 As a result of the detachment of the thoracic side-pieces, the 

 front legs and the structures connected with them are disjoined 

 from the notum, as shown in 

 Fig. 332, and act in connex- 

 ion with the head, while the 



r 



dorsal portion of the segment 

 remains attached to the great 

 thoracic mass. The head is 

 quite free from the thorax 

 and very mobile ; the upper 

 organs of the mouth the 

 labrum and the mandibles 

 are not subject to modifica- 

 tions equal to those exhibited 

 by the maxillae and lower FlG ; ^--Tenthredo, with head fully ex- 



* tended : a, pleuron ; b, pronotum ; c, 



lip, which parts in the bees membrane ; d, mesonotuw. 



are prolonged to form a suc- 

 torial apparatus that may exceed in length the whole body of 

 the Insect. The mandibles remain cutting or crushing imple- 

 ments even when the maxillae and lower lip are modified to 

 form a suctorial apparatus of the kind we have mentioned ; so 

 that in the higher forms ants, bees, and wasps the mouth- 

 pieces are completely differentiated for two sets of functions, one 

 industrial, the other nutritive. 



Behind the head there is a large consolidated mass represent- 

 ing the thorax of other Insects, but made up, as we have already 

 indicated, in an unusual manner, and which therefore may be 

 called by a special name, the alitrunk (Fig. 333). The pronotum 

 forms the anterior part of the alitrunk, with which it is usually 

 very closely connected, being indeed frequently immovably soldered 



