TRACHEARI.E. 
471 
or rounded, and generally soft : in some males, it is narrowed behind into a cylindrical tail. The 
number of eyes is either two or four, and even six, according to Muller. 
EpJais, I,atr., has the chelicerse terminated by a moveable daw. A. extendens, Fabr. 
Hydrachna, Latr., has the mouth composed of plates, forming a projecting 
sucker, and the palpi have a moveable appendage beneath the extremity. 
A. geographicus, Fabr., A. globator, Fabr. 
Lhnnochares, Latr., has the mouth sucker-shaped, but the palpi are simple. 
A. aquaticiis, Linn. [Other species of these water-mites have been described 
by M. Theis, in the Annales des Sci. Nat. for 1832]. 
[From the very valuable discoveries lately made by M. Dugfes, it appears 
that these water-mites undergo metamorphoses, accompanied by a complete 
change of form, the larvae having a very large head and six legs, whilst the 
pupae are inactive, attaching themselves, by a single pair of legs, to the 
bodies of other aquatic insects, and consisting, as it were, simply of an oval 
bag with a narrow neck, the insect in this state having been formed, by 
M. V, Audouin, into the genus Achlysia, and specifically named A. Dytici, from taking up its residence beneath 
the elytra of the Water Beetle They also attach themselves to the slender filaments com- 
posing the tails of the Water Scorpions (Nepa and Ranatra).] 
Other Mites {Micropthira, Latr.) differ from all the foregoing, in having six legs. They are all parasites. 
Caris, Latr., has the sucker and palpi distinct; the body rounded, very flat, and covered with a scaly skin. C. 
vespertilionis, Latr. On Bats. 
[M. V. Andouin has figured an insect which he considers may be identical with Caris vespertilionis, in the 
Annales des Sci. Nat., 1832 ; and which, notwithstanding its possessing only six legs, he considers as more pro- 
perly belonging to the genus Argas.] 
Leptus, Latr., has also a sucker and palpi, but the body is soft and ovoid. A. autumnalis, Shaw (Misc. Zool., 
vol. ii. pi. 42), is very common, in autumn, upon grass and other herbage. They crawl upon our bodies, and in- 
sinuate themselves into the skin at the roots of the hairs, occasioning as painful an irritation as the itch. [It is 
the well-known Harvest Bug], but it is so minute as rarely to be observed. 
The other species are found upon different insects, and enter into the division of the Trombides hexapodes of 
Hermann. T. inseetorum, Herm., T. Libellulce, Herm., T. Culicis, Herm., &c. 
[Aclysia, And., here placed by Latreille, is now proved to be the immature state of Hydrachna.] 
Atoma, Latr., has neither suckers nor palpi visible ; the mouth consists only of a small orifice, situated upon the 
breast ; the body is soft, oval, with the feet short. Acarus parasiticus, Herm. 
Ocypete, Leach, belongs to this section, from the number of its legs ; but, according to him, it has mandibles. 
0. rubra, Leach. Upon Tipulae. 
[From the recent observations of Audouin, Dug^s, and others, it seems questionable whether this terminal sec- 
tion of the Mites can be retained, consisting, as it is now supposed to do, entirely of the young states of various 
groups of Acaridae.] 
[The Senator Van Heyden has lately published a distribution of the Acaridae in the Isis; and many very minute 
species are figured in the continuation of Panzer’s Fauna Inseetorum Germanise, by Herrick Schaffer, distributed 
into many new genera. At the same time, M. Dugfes, in his more elaborate and complete memoir, published in 
the Annales des Sci. Nat., has revised the entire group, dividing it into numerous genera, arranged into the fol- 
lowing groups:— 1. Trombidiei; 2. Hydrachniei; 3. Gamasei; 4. Ixodei ; 5. Acarei; 6. Bdellei; and, 7. Oribatei.] 
Fig’. 41. — a, Hydrachna globulus ; b, magni- 
fied j c, youiig larva ; d, pupa. 
THE THIRD CLASS OF ARTICULATED ANIMALS FURNISHED WITH 
ARTICULATED LEGS,— 
INSECTS (Insecta),— 
Which have articulated legs, a dorsal vessel occupying the place of the vestige of a 
heart, but without any branch for circulation * ; which respire by means of two principal 
♦Anatomists are divided in their opinion as to the nature of this 
organ, many regarding it as a distinct heart, whilst others (including 
Cuvier, whose opinion appears to have been fully confirmed by the 
researches of M. de Serres, inserted in the Mhnoires du Mus. d'Hist. 
Nat.) deny it this quality. Some recent observations appear to esta- 
blish the existence of several small vessels, but besides that, this cir- 
culation must be very partial, as Insects differ materially from the 
Crustacea, the blood not returning to the heart. According to Herold, 
as quoted by Strauss {Bulletin, de Univert Ferussac), the dorsal vessel 
is the true heart of insects, being, as in the higher animals, the loco- 
motive organ of the blood, which, instead of being contained in vessels, 
extends through the general cavity of the body. This heart occupies 
the entire length of the bulk of the abdomen, and terminates ante- 
riorly in a single artery, which is not ramified, and which carries the 
blood to the head, whence it returns to the abdomen by the mere 
effect of its accumulation in the head, to re-enter the heart ; and it is 
in this that the entire circulation of the blood of insects consists, and | 
which are consequently destitute of veins. According to M. Stratss, j 
