by DAVID J. CURTIS.
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Opiliones Notes
by DAVID J. CURTIS. |
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Original references to Nemastoma lugubre have been retained, although they should be correctly referred to as Nemastoma bimaculatum, as justified in the footnote to the paper Curtis, D.J. (1970) Comparative aspects of the fine structure of the eyes of Phalangida (Arachnida) and certain correlations with habitat. J. Zool., Lond., 160: 231-265.
Whilst carrying out an electron microscopical investigation into the eyes of several species of harvestmen at the Department of Zoology, University of Liverpool, I had the opportunity of observing the habits and habitats of these animals in the field. The structure of these eyes was studied in the last century principally by Purcell (1894). More recently, however, Juberthie (1964a,b) has studied phalangid eyes, among other aspects of the animal's biology. He showed the occurrence of regression of the visual system in different groups of phalangids, depending on their habitats.
Curtis (1968,1969) describes the fine structural features of the eyes of seven species and shows how beautifully adapted they are to their function. The lens is a bi-convex cuticular thickening on top of a glassy body of columnar lentigen cells. These cells are longest in Mitopus morio (Fabr.) and Opilio parietinus (Deg.) which live mainly in the upper reaches of the field layer (Todd, 1949; D.J.Curtis, personal observations) and on branches etc. and only slightly shorter in Oligolophus agrestis (Meade) which also lives in the upper regions of the field layer. In Oligolophus tridens (C.L.K.) which lives in the middle of the field layer amidst grass stems etc. the lentigen cells are of medium length, approximately one-third that of the former species. Nemastoma lugubre (Muell.) lives on the ground layer, in or just on the top layer of soil and shows the shortest lentigen cells, barely larger than their nuclei.
The depth of the retina and the length of the rhabdoms, which are the primary photoreceptors at the centre of each of the hexagonally packed retinulae, also show variations in length which may possibly be correlated with environment. In Nemastoma lugubre the retina is very shallow and backed by a cellular sheath, and the rhabdoms are disorganised and contorted. These features, most noticeable in the electron microscope, probably indicate that the eyes of Nemastoma lugubre have undergone a certain degree of regression. This is correlated with the habitat in which the animal lives. Juberthie (1964a,b) states that cavernicolous species show the greatest degree of regression, while epigeal species have the most well-developed eyes. Intermediate degrees of regression occur in species living below stones, amidst mosses etc. Thus Nemastoma lugubre shows the degree of regression in keeping with its habitat, and the differences between the other species may possibly be explained in similar terms. Other interspecific differences, however, are more likely to have a purely physiological basis or merely to represent anatomical differences.
I am grateful to colleagues at the Department of Zoology, University
of Liverpool, especially Drs. P.G.Pearson and C.L.Smith and Mr. R.Dixon,
as well as my wife, for advice and assistance.
CURTIS, D.J. 1969: The fine structure of photoreceptors in Mitopus morio (Phalangida). J. Cell Sci. 4 : 327-351.
JUBERTHIE, C. 1964a: Récherches sur la biologie des Opilions. Ann. Spéléol. 19 : 5-238.
JUBERTHIE, C. 1964b: Récherches sur la biologie des Opilions. D.Sc. thesis, University of Toulouse.
PURCELL, F. 1894 Über den Bau der Phalangidenaugen. Z. wiss. Zool. 58 : 1-53.
TODD,V. 1949 The habits and ecology of the British harvestmen (Arachnida,
Opiliones), with special reference to those of the Oxford district. J.
anim. Ecol. 18 : 209-229.
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