294 Alexander Petrunkevitch 



jumping spider can be taken unawares by an attacking enemy. 

 We know besides from the behavior of the spider that when- 

 ever an insect or anything else approaches it from the side, it 

 immediately turns toward the intruder as though with the 

 desire to see it better by using its front eyes. 



Lycosa nidicola is a spider that lives on the ground under 

 stones, making excursions in the grass. Its manner of walking 

 like that of all ground spiders, is distinctly straight forward and 

 we find that the largest eyes, the posterior middle eyes, are so 

 situated as to guard the front of the animal. In the projection 

 on the horizontal (Fig. 2) plane they together cover a field of 48°, 

 I. e., considerably less than the four eyes of the front row, which 

 cover all together a field of 77°. Between the eyes of the second 

 and those of the third row there is an unprotected area of 

 about 7°, or remembering that the drawing is considerably 

 enlarged, we may say that an object i cm. sq. will be invisible 

 within the space of these 7° as soon as it is farther than 8 cm. 

 from the spider. The presence within this area of a spider of the 

 same species could be already noticed at a distance of about 25 

 cm., quite sufficient to protect against sudden onslaught. The 

 posterior side eyes which are second in size, guard the spider at 

 the sides and back. Thus the entire eye-group covers about 253° 

 of the horizon and leaves unprotected a space on top and at the 

 back. 



In Heteropoda (Fig. 3) the largest eyes are the posterior side eyes 

 The four front eyes cover a field of 145°. Between them and the 

 posterior side eyes there is an unprotected area similar to that in 

 Lycosa, of about 10°. Or since an adult of Heteropoda covers 

 with extended legs about 8 to 10 cm. in each direction, a spider 

 of the same species approaching it within this unprotected 

 area, would become visible at a distance of about 40 cm. The 

 sides are therefore very well guarded especially when we consider 

 that the largest eyes are used in their protection. The eyes of the 

 front row together with the posterior side eyes cover with the inter- 

 ruption mentioned, fully 267°. The dorsal surface of this spider 

 is extraordinarily well protected as compared with the two pre- 

 ceding spiders. There remains an unprotected field of about 



