122 
THE 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 
F1 g. 139.— 
Mouth-parts 
of Bug. (Af¬ 
ter Muhr.) 
Fig. 140, a .—Head of an heter- 
opterous insect. 
Fig. 140, b. — Head 
of an homopterous 
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 Homoptera , includes some forms that 
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