344 ZOOLOGY. 
Cuass VI.—INsEcTA. 
General Character of Insects.—The triregional division 
of the body is better marked in the genuine winged insects 
than in the Myriopods and spiders. They usually have com- 
pound as well as simple eyes; usually two pairs of wings; 
three pairs of thoracic legs; often a pair of jointed abdomi- 
nal appendages, besides an ovipositor or sting which mor- 
phologically represents three pairs of abdominal legs. 
Order 1. Thysanura.—The spring-tails (Podwra) and 
bristle-tails (Zepisma) represent this group. They are wing- 
less, with some affinities to the Myriopods; and the typical 
form Campodea (Fig. 319) is regarded as the ancestral form 
of the six-footed insects, as it is a generalized 
. type, and forms like it may have been the 
earliest insects to appear. 
In Podura, the spring-tail, and also in 
Smynthurus (Smynthurus quadrisignatus 
Pack., Fig. 317), the characteristic organ is 
a forked abdominal appendage or “‘spring,” 
held in place by a hook; when released the 
spring darts backward, sending the insect 
Fig. 317.—Smyn- ; = * 
eats, a aprite- high in the air. 
a, Sees Our commonest Poduran is Tomocerus 
plumbeus Linn. (Fig. 318), found all over the northern 
hemisphere, in North America and Europe. The snow-flea, 
Achorutes nivicola Fitch, is blue-black, and is often seen 
leaping about on the snow in forests. 
The Podurans belong to the suborder Collembola; the 
higher forms, which bear a greater resemblance to the larve 
of Neuropterous insects and to the young cockroach, are 
the Cinura.  Scolopendrella, with its well-developed ab- 
dominal legs, represents the subclass Symphyla. 
In the group Cinura there is no spring, but the tail ends 
in two or three bristles; and in Machilis, the highest form, 
there are compound eyes. In all there are jointed abdominal 
appendages, which structures are unique among Hexapodous. 
insects. Campodea staphylinus (Fig. 319) is a small white 
