36 INTRODUCTION TO EVOLUTION 



parts, possessed by other moths, are in reality "made over" from the biting 

 and cutting type. 



A third modification is possessed by the true bugs (Order Hemiptera). 

 They have a proboscis somewhat like a hypodermic needle which they 

 thrust through the skin of plant or animal to withdraw underlying juices 

 (Fig. 3.9). Though the most notorious member of the group is the bed- 

 bug, by far the larger number of true bugs suck the juices of plants and of 

 other insects rather than the blood of vertebrate animals. Unlike a hypo- 

 dermic needle the proboscis is not thrust through the skin by sheer force 

 applied to it. The creature wielding it is too tiny for that. Rather, a hole is 

 drilled for its insertion, the drilling being done by two pairs of sharp, 

 piercing bristles (Fig. 3.9). The innermost pair of these is formed from the 

 1st maxillae, hollowed out on their adjoining faces to form the walls of two 

 tubes. Plant and animal juices are sucked through the larger, dorsal tube; 

 saliva may be forced outward into the puncture wound through the 

 smaller, ventral tube (Fig. 3.9). On either side of the 1st maxillae are the 

 mandibles, also modified to form piercing bristles. In drilling the hole 

 the four bristles slide up and down independently, the mandibular pair be- 

 ing the more active in the process. This hypodermic arrangement is en- 

 cased for a portion of its length in a rostrum or beak formed of the 2nd 

 maxillae (Fig. 3.9). 



The two-winged flies, the housefly being the most familiar example, 

 have a proboscis formed from labrum, hypopharynx, and labium (2nd 

 maxillae) (Fig. 3.9). In some flies the proboscis terminates in a pair of 

 broad, soft pads (labefla) pierced by many pores which function in 

 "sponging up" liquids; in biting flies the proboscis is modified for piercing. 



We see, then, how a set of "standard parts" (labrum, mandibles, hypo- 

 pharynx, 1st and 2nd maxillae) have been modified to serve such diverse 

 food habits as cutting and shredding plant tissues, sucking nectar from 

 flowers (by two different types of mechanism), piercing the skin to suck 

 juices of plants or animals, and gathering liquid from the surfaces of food 

 particles. Why are such diverse mechanisms based upon the same under- 

 lying pattern? Evidently the basic pattern of mouth-part structure was in- 

 herited from an ancestor shared by all these modern insects. As noted 

 above, the mandibulate or cutting mouth parts represent the type from 

 which afl the others are believed to have arisen through adaptive radiation. 



Serial Homology 



Thus far we have spoken of the homology of an organ in one animal 

 with an organ in another animal. We have said, for example, that the wing 



