100 BRISTLE-TAILS AND SPRING-TAILS. 
rated by a deep suture from the clypeus, and is trapezoidal in form. 
The mandibles and maxille are long and slender, and buried in 
the head, with the tips capable of being extended out from the 
ring surrounding the mouth for a very short distance. The man- 
dibles (md, Fig. 30) are like those of the Neuroptera, Orthoptera 
and Coleoptera, in their general form, the tip ending in from three 
to six teeth (three on one mandible and six on the other), while 
below (Fig. 41, md) is a rough, denticulated molar surface, where 
the food seized by the terminal teeth is triturated and prepared to 
be swallowed. Just behind the mandibles are the maxilla, which 
are trilobate at the end, as in the three orders of insects above 
named. The outer lobe, or palpus, is a minute membranous tuber- 
cle ending in a hair (Fig. 31, mp), while the middle lobe, or galea, 
is nearly obsolete, though I think I have seen it in Smynthurus 
where it forms a lobe on the outside of the lacinia. The lacinia, : 
or inner lobe (Fig. 31, lc; 32, the same enlarged), in Tomocerus 
consists of two bundles of spinules, one broad like a ruffle, and the 
other slender, pencil-like, ending in an inner row of spines, like _ 
the spinules on the lacinia of the J apyx and Campodea, and, more 
remotely, the laciniæ of the three orders of insects above referred 
to. There is also a horny, prominent, three-toothed portion (Fig. 
31, g). These homologies have never been made before, but they 
seem natural, and suggested by a careful examination and com- 
parison with the above-mentioned mandibulate insects. 
The spring consists of a pair of three-jointed appendages, with 
the basal joint soldered together early in embryonic life, while the 
two other joints are free, forming a fork. It is longest in Smyn- 
thurus and Degeeria, and shortest in Achorutes (Fig. 36, b), where 
it forms a simple, forked tubercle; and is obsolete in Lipura, its 
place being indicated by an oval scar. The third joint varies in 
form, being hairy, serrate and knife-like in form, as in Tom 
cerus (Fig. 30, a), or minute, with a supplementary tooth, ag 
Achorutes (Fig. 36; ¢). This spring is in part homologous with 
the ovipositor of the higher insects, which originally consists of 
three pairs of tubercles, each pair arising apparently from the 
seventh, eighth, and ninth (the latter the penultimate) segments 
of the abdomen in the Hymenoptera. The spring of the Podura 
seems to be the homologue of the third pair of these tubercles, and 
is inserted on the penultimate segment. This comparison I have 
“been able to make from a study of the embryology of Isotoma. 
