james spencer's picture

Spider sp

Observed: 14th October 2011 By: james spencerjames spencer’s reputation in Invertebratesjames spencer’s reputation in Invertebratesjames spencer’s reputation in Invertebrates
spider 2
Description:
Identifications
Species interactions

No interactions present.

Comments

Syrphus's picture

I follow your argument,

I follow your argument, James, but it is faulty logic. If you pick a spider at random (taking your figures for the sake of argument) there is a 22% chance that it will be saeva and 78% duellica.

However, we are now looking at one spider in the pic, and the probabilities are that is is either 100% saeva or 100% duellica. You can only decide which it is by reference to the distinguishing features, which are invisible without a microscope. The ID should then be to genus only.

If you extend your reasoning, every Tegenaria pic from that area would have to be identified as 'likely to be duellica', despite the fact that you would be wrong 1 time in 5, and you would never know which 1 in 5 was the wrong one. In other words, you could never get a species ID from the pic, which is what this process is all about.

In other groups, and with some specialist knowledge, you can apply the sort of reasoning you have tried (I have done so myself, and you may want to argue with these instances - but please don't do that on this thread!), but justifying it explicitly on the basis of habitat, phenology, hints in the picture, and local knowledge of the species, etc.

Murdo

TRY

recording wildlife with The Recorder's Year on www.hbrg.org.uk/TRY.html.

james spencer's picture

Many thanks Murdo - that is a

Many thanks Murdo - that is a fair piece of observation and I was applying flawed logic in the wish for a field ID.

regards

James