JOURNAL OF THE ROYAL MICROSCOPICAL SOCIETY. DECEMBER, 1878. I.-On a Species of Acarus, believed to be new to Britain. (Read before the ROYAL MICROSCOPICAL SOCIETY, 1st May and 13th December, 1878.) PLATE XVI. Cheyletus venustissimus. In the beginning of January last (1878) I found this species, which I believe has not hitherto been detected in Britain, in a stable; near Tamworth I noticed one or two minute red specks running rapidly over the fodder. I secured three or four for examination, and I then ascertained that it was the Cheyletus venustissimus of Koch;† that author, in his work published in 1839, describes and figures the species in a manner sufficient for identification, although somewhat wanting in detail; he states that it was then found in the neighbourhood of Regensburg and in Rhenish Bavaria, and was somewhat rare. The species is not noticed by Walckenaer and Gervais, and I am not aware that any subsequent author has noticed it, except by stating that Koch gives it as one of the genus; as Koch's notice is not very full and does not mention the larva, pupa, &c., a description may possibly be acceptable. EXPLANATION OF THE PLATE, Cheyletus venustissimus. FIG 1.-Under side of female x about 70 (the imbrication of the hairs is somewhat exaggerated); a, horseshoe-shaped ridge round anus. 2.-Rostrum and palpi x about 200; a, first joint of palpus; b, second ditto; c, falx; d, third joint of palpus. 3.-Larva. 4.-Foot highly magnified. * Subsequent to the reading of this paper Mr. McIntire was good enough to show me a slide of unnamed Cheyletus captured by him. It has been mounted some years, but I think it quite possible that it may have been this species. + Deutschland's Crustaceen,' Heft xxiii. p. 22. VOL. I. Z The species, while thoroughly preserving all the above-named characters of the genus, which are very marked, is, as far as those characteristics will allow, a complete contrast to that last described, its red or orange colour, lighter build, and longer legs, evidently adapted for speed and activity, making it a remarkably differentlooking creature. The median stripe in this species is very conspicuous, and extends from the widened oesophagus to near the anus, and is formed by the alimentary canal showing through the skin; the normal shape of the stripe is that of an hour-glass, and the colour is opaque white; the shape of the marking, however, varies considerably, both in different individuals, and in the same individual from time to time, according to the position of the food in the canal, and the quantity contained. The body on each side of the median stripe is red or orange, the palpi, legs, and œsophagus clear light yellow or yellowish white. The shape of the body, as in all the Cheyleti, is diamond or coffin shaped, with the anterior part of the diamond shorter than the posterior, and the front and anal angles rounded off, the points of the lateral angles being formed as in the last species; but the body in Venustissimus is longer in shape, being nearly twice as long as it is broad, without the rostrum, and much flatter, the thickness not being above a sixth of the width at the widest part. The diamond shape is not so conspicuous as in the last species; the body is marked by five slight constrictions, which produce a somewhat scalloped outline; the constriction between the cephalothorax, which occupies over two-thirds of the whole bulk, and the abdomen, is only slightly marked on the upper side or at the edges, but on the under surface it is marked by a very deep depression, widest on the ventral surface, and narrowing inwards; the raised median portion of the under surface of the cephalothorax slopes suddenly down from nearly opposite the insertion of the fourth pair of legs to the bottom of this depression, forming a blunt oblique triangle. The anus is placed in the centre of a strong horseshoe-shaped ridge, with the points drawn out laterally, which is marked with numerous folds, and is probably contractile. The skin of the legs, palpi, and under side of the body is very finely striated; the striation is, however, not nearly so strong as in the last species, and on the body the colour renders it difficult to see. The anterior pair of legs are far the longest, and very thin and fine; these legs constitute the most marked characteristic of the species, being strikingly different from the comparatively short firm legs of the other Cheyleti, particularly the last species. The coxa is short, stouter than the other joints, and is somewhat angled on the posterior side. The second joint (or trochanter) is long and straight; it is decidedly striated. The third joint (the femur) is not much above half the length of the second. The fourth joint is nearly as long as the second, and has the inner side of the posterior articulation projecting. The tarsus has a slight shoulder a little beyond the articulation, from this point it narrows in and becomes very fine; the tarsus is the longest joint of the leg; the tarsal sucker of this particular leg is small and fine, and the claw very small and difficult to make out. The use of this first pair of legs, and the distribution of the hairs on all the legs, are mentioned below. The second, third, and fourth pairs of legs, although longer than in the last species, do not vary from the usual type of the genus sufficiently to render it necessary to describe them. The sucker and claw and its supports are given in the drawing (Plate XVI., Fig. 4). There is a row of seven or eight strong hairs on the upper side of the body, a little within the edge, the first three near together; each coxa bears a strong short curved hair pointing downwards and towards the foot. The first pair of legs have on the second joint two longer straight hairs a little beyond the middle, one on the upper and one on the under side; on the third joint two similar on the upper side, a little before the middle; on the fourth joint two similar at the commencement on the outer and upper side, and one short one on the hinder and under side, one short and one long about the middle of the joint, and one very long strong one on the upper and outer front end of the same joint, this is the longest and strongest hair on the creature; on the tarsus there is a similar hair, not quite so long as the last, springing from the upper surface at the before-mentioned shoulder. All these hairs are conspicuously imbricated, like those of the Indian bat; on each side of the point of the tarsus is a long strong hair, and in the centre is a shorter and finer one; these hairs are closely jointed or ringed, but are not imbricated. The second pair of legs have a pair of hairs on the third joint, and two pairs on the fourth joint, all imbricated; one small one half-way down the tarsus, and two pairs at the extreme end, all small and plain. The third pair of legs have a pair of hairs, and the fourth pair of legs one hair on the second joint, the hairs on the remaining joints being like those of the second pair of legs. The palpi in this species are without teeth on the falces, otherwise like the last species but slighter (Plate XVI., Fig. 2). It is well known that in the acari the first pair of legs are modified palpi, and M. Robin has remarked that in some instances they appear not entirely to have lost the office of organs of touch; This joint is called the tibia by Nicolet. Robin rejects this name, and calls it "la jambe," following Savigny in reserving tibia for the lower half, where this joint is divided into two shorter ones. |