Art. XVII.—Description of a New Species of Migas, with Notes on its Habits.
[Read before the Otago Institute, 13th May, 1890.]
Gen. Migas, Koch.
Migas sandageri, sp. nov.
Femina: Length about 9mm.
Cephalothorax and falces brown; legs and palpi brownish-yellow, with wide dark-brown flecks and annulations; sternum, labium, and maxillæ yellow suffused with brown; abdomen dark-brown, darker above than below, and speckled with
minute flecks of a dirty skin-colour. In places these flecks coalesce into narrow bars, transverse above and below, and oblique at the sides; but both flecks and bars are invisible to the naked eye. Spinners and branchial opercula pale-yellow.
Cephalothorax about 1mm. longer than broad, forming in outline an oval truncated in front; from the fovea, which is deep and semicircular, with the curve directed forwards, radiate eight grooves, three running down each side, and two down the posterior slope. In front of the fovea are two yellow spots, one on each side of the thoracic median line, from each of which springs a stout bristle. There are also bristles on the ocular area; but the rest of the surface is quite glabrous. The caput is moderately prominent.
Both rows of eyes curved forward, the posterior more curved and shorter than the anterior row; front central eyes round, of a dark colour, placed on black prominences, and nearer to each other than to the laterals of their own row; the latter of a pale colour, considerably larger than the centrals, oblong, each placed obliquely in front of a black tubercle behind which are situated a central and a lateral of the posterior row; eyes of the latter row minute, subequal, longer than wide, opalescent (the centrals brilliantly so), and posited obliquely, the centrals looking inwards and the laterals outwards, the former being very distant from each other, and each quite near the lateral on its own side; posterior laterals nearer to anterior laterals than the latter are to the fore centrals.
Falces sparingly hairy at the fore extremity, prominent, powerful, knee-shaped; groove toothed, teeth on outer side small and subequal, those on the inner side large and unequal, the one nearest the fang the largest, and the basal one the smallest.
Maxillæ very divergent, and of the same width through their whole length, outer side longer than inner, the latter furnished with a long fringe; the inner half of the inferior surface studded with short tooth-like spines.
Labium about half as long as the maxillæ, slightly convex, triangular in outline, rounded at the apex, the anterior part of it studded with spines like those of the maxillæ.
Sternum ovate in outline, truncated in front, pointed behind, sinuated at the sides, having two small depressions, one on each side, not far from the margin, between the 2nd and 3rd pair of legs, and sparsely hairy.
Abdomen oblong-oval, convex, and copiously furnished with short stoutish hairs; spinners compactly grouped, the inferior pair slender and of moderate length, the superior pair stout and twice as long as the inferior.
Both in shape and armature the legs and the palpi are like those of M. distinctus.
The genital aperture is a simple transverse slit.
This spider bears a resemblance to M. paradoxus, Koch, but differs from the latter in having its front row of eyes curved forward and its hind laterals closer to the fore laterals than these are to the fore centrals, in the shape of the joints of the legs and palpi, in the denticulation of the claws, in having no distinct abdominal pattern, in the length of the superior pair of spinners, in not having its sternum “äusserst fein netzartig,” and in the absence of “ein den Schenkeln des vierten Paares entsprechenden Eindruck.”
Hab. Mokohinou Islands, Sandager.
I have much pleasure in associating this Migas with the name of Mr. F. Sandager, who is the author of some valuable papers on the fauna and flora of Mokohinou Islands, and to whom I am indebted for my examples.
This interesting little spider builds its nest on the bark of trees (Coprosma, Cordyline, and Fagus). There are generally several nests on each tree, the lowest being at least a foot from the ground, and the highest as high as the base of the larger branches. The larger nests are for the most part built in the hollows, and the smaller ones on the more even surface of the bark. On this account the latter present the appearance of small prominences or knots in the bark. The lid is round, and in all my specimens hung on the outer side of the tube, which is always lower than the inner side by the diameter of the lid. This build of the nest makes the lid when closed lie in the same plane with the bark of the tree—an arrangement that seems designed to conceal the entrance to the nest from the enemies of its occupant. The tubes in my possession vary from 3mm. to 5mm. in diameter, are from three to four times as deep as wide, and are thickly lined throughout with web. The shallowness of the nest is no doubt of great advantage to the spider, for, should the entrance be discovered by an enemy, the tenant is enabled to reach the lid in an instant, and, by thrusting the claws of its powerful fore-legs into the web lining the under-surface, to hold the lid down so firmly as to prevent the ingress of its would-be devourer. This is the method adopted by all trapdoor spiders to resist the attempts of their enemies to open the door of their nest. But the most wonderful feature of the nest of Migas sandageri is the marvellous resemblance of its exterior surface to all the details of the bark on which it is built. The colour, the variations of colour in different trees, the scales, the very rugulosities of the bark, are reproduced with a fidelity that would do credit to an artist. A more perfect example of protective mimicry I have
never met with. It has been urged against the theory of sexual selection that it postulates the existence of an æsthetic sense in the lower animals; but, when one sees an animal so lowly organized as the spider able to weave for its protection a well-shaped tube, to make and attach to it by a strong flexible hinge a perfectly-fitting lid, and then to cover the entire exterior surface of both tube and lid with materials so selected and adjusted as to produce an exact limitation of the varying surface of the objects on which it builds, one does not feel disposed to attach much weight to the objection.
Explanation Of Plate Xx.
a. Natural length of spider.
b. Migas sandageri.
c. Under-side of four joints of a leg of the first pair.
d. Left palpus.
e. Piece of bark with nest, the latter purposely made less inconspicuous than in nature.