NOTES ON AUSTRALIAN ARANEID/E — RAINBOW. 215 



naturally arises as to the reason for the presence of this ribbon. 

 It cannot be for special protection, because the web is usually 

 built in an open space, where owing to the animal's colour and 

 the conspicuous position it takes up at the centre of its web, it 

 can be easily seen. A bird or predatory insect in quest of such 

 prey (for the Argiopids are often found in wasps' nests) would 

 not be likely to miss it. McCook expressed the opinion that the 

 ribbon is introduced for the purpose of strengthening the shield, 

 but since all the species do not construct a shield or screen, that 

 cannot fully explain the reason for its presence. Personally, I 

 am inclined to the opinion that it acts as a support to the central 

 space and hub and also that it materially assists in bearing the 

 weight of the spider's body ; where a shield is constructed, the 

 zigzag ribbon woidd, doubtless, be beneficial. The central space 

 and hub of the web of Argiope is very fragile and delicate, whilst 

 the spider, in proportion tu it, especially when gravid, is large 

 and heavy. 



The spider, like all orb-weaving species (with the exception of 

 those which roll or fold leaves, whose habits will be referred to 

 later on) when at rest takes up her position at the centre or hub, 

 and always head downwards ; but the Argiopea* differ from all 

 other Argiopidse in the disposition of their legs, for whereas orb- 

 weavers in general spread their eight ambulatory limbs well out, 

 Argiope, on the other hand, always rests with hers arranged in 

 pairs, well extended, and so approximated that they describe the 

 letter X. A, cetherea and A. cemula always dispose theirs in pairs 

 over the zigzag ribbons forming the St. Andrew cross design. 



The general structure of an Argiope web is very similar to that 

 of the typical orb-weaver ; it varies in dimension, according to 

 the size of the architect, and sometimes according to environment; 

 but whatever the size or surroundings may be, the zigzag band 

 of silk on stabilimentum is always present, hence the naturalist 

 can always tell when he sees one of these snares (even though the 

 architect be absent) the genus to which the latter belonged. It 

 is interesting to note the modus operandi of working-in the 

 ribbon. The main structure of the web with its supports con- 

 sisting of outer lines or groups, radii and concentric rings having 

 been completed, A. cetherea drops down to a point a little below 

 the centre of the snare and takes up its position between two 

 rays running in an oblique direction towards the central point of 

 the hub ; this she ascends, discharging silk from her spinnerets 

 as she does so, and as all are at work voiding simultaneously, ii 

 follows that a broad ribbon is the result. The zigzag effect is 

 caused by the animal swinging its abdomen from side to side as 



