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 

 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 



bv tbp maxillae and lower ^^^°- ^^'^■ — Tenthredo, with head fully ex- 

 Oy tne maXUiae anu lOVVei tended, a, pleuron ; J, pronotum; c, 



lip, which parts in the bees membrane ; cl, niesonotum. 



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 



