d, and greater in
extent than the epicranio-occipital region. The antennae are inserted
high up on the head, next the ocelli, falling down over the clypeal
region. The clypeus, however, is merged with the epicranium, and the
usual suture between them does not appear distinctly in after life,
though its place is seen in figure 167 to be indicated by a slight
indentation. The labrum is distinctly defined by a well marked suture,
and forms a squarish, knob-like protuberance, and in size is quite large
compared to the clypeus. From this time begins the process of
degradation, when the insect assumes its Thysanurous characters, which
consist in an approach to the form of the Myriopodous head, the front,
or clypeal region being reduced to a minimum, and the antennae and eyes
brought in closer proximity to the mouth than in any other insects."

Sir John Lubbock has given us an admirable account of the internal
anatomy of these little creatures, his elaborate and patient dissections
filling a great gap in our knowledge of their internal structure. The
space at our disposal only permits us to speak briefly of the
respiratory system. Lubbock found a simple system of tracheae in
Smynthurus which opens by "two spiracles in the head, opposite the
insertion of the antennae," _i. e._, on the back of the head. (Von Olfers
says that they open on the prothorax.) Nicolet and Olfers claim to have
found tracheae in several lower genera (Orchesella, Tomocerus, and
Achorutes and allied genera), but Lubbock was unable to detect them, and
I may add that I have not yet been able after careful search to find
them either in living specimens, or those rendered transparent by
potash.

Having given a hasty sketch of the external aspect of the Poduras, I
extract from Lubbock's work a synopsis of the families and genera for
the convenience of the student, adding the names of known American
species, or indications of undescribed native forms.

SMYNTHURIDAE.--Body globular or ovoid; thorax and abdomen forming one
mass; head vertical or inclined; antennae of four or eight segments. Eyes
eight on each side, on the top of the head. Legs long and slender.
Saltatory appendage with a supplementary segment.

Smynthurus. Antennae four-jointed, bent at the insertion of the fourth,
which is nearly as long as the other three, and appears to consist of
many small segments. No conspicuous dorsal tubercles. (In this country
Fitch has described five species: S. arv