Fields of View of the Eyes Of Primitive Jumping Spiders

Two species from the sub-family Spartaeinae were obtained. Portia fimbriata, originally from Queensland, Australia, were kindly supplied by Dr Robert Jackson of Christchurch, New Zealand. A single female of Spartaeus spinimanus, from Singapore, was collected by Dr David Blest of the Australian National University, Canberra. In addition, specimens of the ‘advanced’ salticid Plexippus sp. were obtained locally in Canberra. Live spiders were anaesthetized with CO₂, and a small stick was waxed to the back to serve as a mount. They were then placed in the apparatus shown in Fig. 1. This consists of a Cardan arm on which is mounted a small telescope, and a light source whose beam is concentric with the viewing beam. The telescope has an acceptance angle of about 2°, which limits the accuracy of the method. The telescope is focused on the spider, and a reflection from the retina of each eye can be seen provided the axis of the telescope is within the field of view of the eye. The Cardan arm is calibrated in latitude and longitude coordinates, and these are used to plot the locations of the edges of the fields of view of the secondary eyes (Fig. 2). The spider was aligned so that the equator of the coordinate system corresponded with the ventral rim of the prosoma carapace. Jumping spider eyes do not have tapeta, but the retinae contain screening pigment which causes them to appear slightly pink. Outside the retina this colour changes abruptly to grey. Additional ophthalmoscopic observations on the secondary retinae, and on the fields of view of the principal (antero-median) eyes were made using an ophthalmoscope described elsewhere (Land 1969b, 1984).

In Fig. 2 the fields of view of Portia and Plexippus are compared. In Portia there is a gap about 20° wide between the fields of view of the antero-lateral (AL) and postero-lateral (PL) eyes. This gap is filled by the strip-shaped field of the posteromedian (PM) eye. In Plexippus there is no gap, and the fields of the AL and PL are contiguous in this region. It appears that the fields of both AL and PL eyes have expanded to fill the gap. In Spartaeus the field of view of the PM eye is very similar to that of Portia but is not so extensive vertically. It stretches from about 20° below the equator to 40° above (60° total) compared with 35° below to 60° above in Portia. The spacings of the receptors in the secondary retinae of Portia were examined ophthalmoscopically. In the AL eye the centre-to-centre separation varied from 0·55° in the acute region near the body axis, to 0·97° near the lateral edge. In the PL eye the separation was nearly uniform at approximately 1·49°. Interestingly, the separation in the PM eyes was intermediate, at almost exactly 1-0°, and uniform. This contrasts with other jumping spiders where estimates for the PM eye, to the extent that they have any meaning, are around 10°. Clearly in the Spartaeinae, the PL eyes are integrated into the overall function of the secondary eye system, whereas in most other salticids they are not.

Unfortunately, the location of the field of view of the PM eyes in Portia does not correspond particularly closely with the PM fields of any of the other hunting spider families that have so far been studied (for maps of these see Land, 1985). In the Lycosidae (wolf spiders) the PM eyes are the largest pair and their fields are also large, extending from directly above the animal to 50° below the equator, and from the front to about 70° lateral. In both the Sparassidae (huntsmen) and Thomisidae (crab spiders) the PM fields are more dorsal and less frontal than in the Lycosidae, but in all these families the region around the dorsal pole is occupied by the fields of the PM eyes, not the PL eyes as in the Spartaeinae. The PM fields in these other families are also much larger than they are in the Spartaeinae although - for what this is worth-they are smaller in the Thomisidae than in the Lycosidae or Sparassidae. The visual fields in the other hunting spider families (Clubionidae and Oxyopidae) have yet to be investigated. Possibly one should also look to the web-building families since Portia, unlike other salticids, does build a web (Jackson & Blest, 1982). However, the optical design of the eyes in most of the web-building families does not permit the accurate mapping of visual fields (Land, 1985). If Portia and Spartaeus are properly regarded as primitive, then we may conclude that the ancestors of the salticids had six functional secondary eyes, like most other spiders, but that the distribution of their fields of view was not like that in any other family that we have good information about at present.

It is a pleasure to thank Dr David Blest for helpful discussions and for providing specimens. This work was done while the author held a fellowship in the Neurobiology department of the Research School of Biological Sciences at the Australian National University, Canberra.