330 THE BIOLOGY OF INSECTS 



Neuroptera, and Hymenoptera, for example — the basal 

 regions of these latter appendages tend to become more 

 closely united together while the lobes become reduced 

 so that the labium is more definitely modified into a 

 " lower lip." 



When we turn from the biting insects to the haustellate 

 or sucking groups, we find that the mouth-parts are modified 

 in ways the most diverse for feeding on liquids. The 

 mandibles may disappear or be reduced to minute vestiges, 

 as in the Trichoptera, Lepidoptera, and many Diptera, or 

 they may persist, modified into sharp piercers as in the 

 Thysanoptera, Hemiptera, and many other Diptera. The 

 maxillae may also be transformed into piercing organs as in 

 Hemiptera and biting Diptera, or become modified into 

 flexible grooved structures acting jointly as the sucking tube 

 wliich is the feeding organ of the Lepidoptera. Among 

 the Diptera the act of suction is performed by the tip of 

 the labium, and the same function is so carried out by the 

 Hymenoptera, an order within which the jaws of the 

 primitive sawflies are essentially of the mandibulate type, 

 while among the highly specialised bees the conjoined 

 elongate and flexible inner lobes of the labium form a most 

 beautifully perfect sucking trunk. All these facts combine 

 to demonstrate that the arrangements of the jaws for sucking 

 which characterise various orders of insects are not only 

 most diverse, but have been independently acquired as 

 modifications, often extreme, from the primitive biting 

 type of insect mouth. 



The wings of insects of different orders also show 

 evidence of modification. Those groups such as Isoptera, 

 Odonata, and some Neuroptera in which the hindwings 

 and fore wings are closely alike, represent generally the 

 primitive undifferentiated condition of insect wings, but it 

 is established by the researches of J. H. Comstock (191 8) 

 and others that among such living insects as the Isoptera, 

 whose hindwings resemble the forewings most nearly in 

 shape and neuration (Fig. 64, h)y this condition is to be 

 regarded as a secondary reversion. Comparison of insect 



