126 AMERICAN SPIDERS AND THEIR SPINNINGWORK. 



II. 



The third group of Orbweavers making vertical webs is composed of 



those which retain the open hub. Among these are our indigenous species 



of the genus Acrosoma. These are found in the neighborhood of 



Orbs with Philadelphia, and are distributed very generally throughout the 



Middle, Northern-Middle, and Southern States, east of the Rocky 



Mountains. The favorite site for their snares is a large open 



space between two bushes or trees, or between the diverging branches of 



the same. I have most frequently found them in the margins of an open 



grove, wood, or forest. As a rule they swing their nets at a considerable 



height, so that one's face comes in contact with them while passing through 



the woods. The foundation lines are frequently of considerable length, four 



or five feet, or even more. The delicate orb swung between them is a very 



pretty sight as one sees it outlined against the sky, showing through the 



vista of the opening trees. (Fig. 111.) 



The three species common to our neighborhood are Acrosoma rugosa, 

 spinea, and mitrata. l They are all characterized by abdomens which present 

 upon the dorsal surface spines of greater or less length and hard- 

 Favorite negg These spines are more decidedly developed upon the first 

 two named species. In the last named species, the Mitred spider, 

 the spinous processes are small, and the integument is not so tough. 



All the three species make substantially, and I might say almost pre- 

 cisely, the same sort of web. The shape of the snare is usually quite orbic- 

 ular, nearly always approaching a circle more nearly than that of most 

 species of Orbweavers. The number of radii is very great, amounting at 

 times to as many as eighty, and the number of spirals is corre- 

 Character spondingly large. The orb itself is not very large, generally 

 r ' being within six inches in diameter rather than above. It re- 

 sults, therefore, that the spiral space presents a remarkably close texture of 

 checkered openings between the cross lines. The free zone, which in the 

 typical Epeira net contains no lines crossing the radii, is always occupied 

 in this genus by the lines of the notched zone, which wind in three or four 

 comparatively widely separated concentrics through the entire free zone. 

 These take the place of the notched zone of the Epeira orb, whose con- 

 centrics are wound close up to the hub. 



The hub, as has been stated, is always open, and within it the spider 

 is usually found hanging with its legs outstretched, grasping the marginal 

 circumference of the hub. Spinea and Mitrata hang in a position closely 

 resembling that of the ordinary Epeira, that is to say, with the head 



1 As I have heretofore shown (Proceed. Acad. Nat. Sci. Philada., 1888, page 5), the names 

 of these species as given by Walckenaer are entitled to priority. They will probably be 

 known respectively as Acrosoma gracilis (rugosa), A. saggittata (spinea), and A. reduviana 

 (mitrata). 



