Tuesday, 4 February 2014

Is this spider male or female?

I promised a while ago to explain how to tell the difference between male and female spiders. It's one of the most common questions I get asked, especially by kids when I point out a mummy or daddy spider!
On a side note, I met so many kids during my garden surveys that were fascinated by the spiders we were finding, and asking really great questions too! It really made my day. If these kids keep on asking questions like this about everything they see around them they're going to make great scientists one day.

Anyway back to sexing spiders...hmmm...

Firstly, as you might know, most females spiders are larger than the males. This is called sexual dimorphism and it is a result of smaller males having an evolutionary advantage (ie. they are able to sneak in and mate with the female, or avoid being eaten by her afterwards!).

Here is a photo I took of a very large female of my study species Nephila plumipes. Can you see the tiny little male on the left?

But sometimes the males of this species are also quite large (comparatively). The photo below is a male (right) with an immature female. This large variation in the sizes of males can happen (in an evolutionary sense) when males have an advantage if they are EITHER large (ie. more likely to out compete a smaller male) or very small.

In some species the males are always the same size, or even larger than the females. This photo shows a pair or Garden orb weavers (Family Araneidae, Genus: Eriophora)

The one on the left is the female and the right is the male. The way I can tell is by looking at the palps, the appendages near the mouthparts. The male has large palps with bulbous ends, he uses these to deposit sperm into the female. In comparison the female has long thin palps. 

Females also have a genital opening called the epigynum near the book lungs on the underside of the abdomen (although this is harder to see). In some Araneidae species there is an interesting protrusion from the epigynum, I'm not sure what this is for. This photo comes from the Cross spiders I was studying in Germany, more on that later.