Sunday, March 10, 2013

Orb Spiders

Lobed argiope (Argiope lobata)

Orb spiders are a group of large, often colourful, spectacular spiders that are represented throughout most of the world. The lobed argiope (brimba kbira tal-widien) is the largest Maltese spider and is often found in valleys and wooded areas. 

Recently another species of orb spider, the banded argiope, was discovered in Malta. It was noted at the Simar and G─žadira Nature Reserves and now seems to have spread to other localities. This is an American species and is not normally found in Europe. The wasp spider is closely related to the lobed argiope. 

It is found throughout Central and Northern Europe, North Africa and parts of Asia. It used to be found in Malta but apparently has become extinct in the Maltese islands. The male spider is much smaller than the female and has no distinctive markings. In the picture he can be seen just above the much larger female spider.

Spiders reproduce sexually. Although fertilisation is internal it is indirect, that is, the sperm is not inserted into the female’s body by the male’s genitals but by means of an indirect stage. 

Before approaching a female the male must ensure that she is of the same species. This is often done by means of complex courtship displays. Sometimes he identifies her by checking the web, which might have characteristics particular to the species. The male argiope spins a web close to that of the female’s and then approaches the female. He then spins a small web in which he places his sperm. 

A female spider may lay up to 3,000 eggs in one or more silk eggs sacs. Some females die after having laid their eggs while others protect the sacs by attaching them to their webs, hiding them in nests or carry them around attached to their body.

Young spiders remain inside the egg throughout their larval stage. They hatch as spiderlings – small, sexually immature but similar to adults. As they grow they moult as their cuticle cannot stretch and cannot accommodate their body as this grows larger. In some species males mate with newly-moulted females which are too weak to be dangerous to the males. 

Ths article was publshed in The Times on 12 August 2013. 

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