122 7WE STUDY OF INSECTS. 



jointed sheath (Fig. 139). Two of the bristles represent the 

 mandibles, and two the maxillae. The sheath is supposed 

 to consist of the labium and the grown-together labial palpi. 

 In their transformation the Hemiptera pass through an 

 incomplete metamorphosis ; the young nymphs resembling 



FIG. i 3 9. 



Mouth-parts FIG. 140, a. Head of an heter- FIG. 140, b. Head 



of Bug. (Af- opterous insect. of an homopterous 



ter Muhr.) insect. 



the adults more or less closely in form, and the wings being 

 gradually developed at successive molts. 



This order includes three well-marked groups, which are 

 ranked as suborders. The first of these, the Heteroptera, 

 includes the true bugs. They are placed first, as we believe 

 they resemble the ancient Hemiptera the first to appear on 

 the earth more closely than the members of either of the 

 other suborders. The second suborder, the Parasitica, in- 

 cludes the lice. These insects are much lower in structure 

 than the Heteroptera ; but we believe that this simplicity 

 in structure is a result of degradation due to parasitic habits, 

 and therefore really represents a later development than 

 that shown- by the Heteroptera. In other words, the lice 

 are probably descendants of some ancient form resembling 

 some of the existing Heteroptera. Among the Heteroptera 

 the bedbug exhibits a similar downward tendency. The 

 third suborder, the Homoptcra, includes some forms that 



