Friday 3 September 2010

Spider chat up lines

ResearchBlogging.orgIn summer and autumn, spiders become more noticeable. The tiny spiderlings born in the spring have now become adults and males are wandering in search of females. One of the most common spider species in the UK is the elegantly marked Linyphia triangularis (female above). This spider makes a sheet web with criss-crossing silk lines over it in low bushes and trees. The spider hangs belly up from the underneath the sheet. Flying insects colliding with the transversal lines  fall onto the sheet, where the spider traps them. Yesterday, while photographing 7-spot ladybirds I came across a pair of L. triangularis. The male made jerky movements with its chelicerae and palps and the female replied.

Male (on the left) and female Linyphia triangularis. Note the male's large fangs on the bottom photo.
Unfortunately, I seemed to have disturbed them and the male retreated, so I didn't see the end of the interaction. Field experiments carried out by Nielsen and Toft showed that male L. triangularis use what are known as 'alternative mating tactics'. Males guard subadult females just prior to their final molt in order to access them while they are still virgin, possibly due to a strong sperm preference which results in males mating with a virgin female fathering most of the female's subsequent offspring. These researchers took advantage of this behaviour and took 56 males and paired them with females in the field. Once this relationship was established they introduced an 'intruder' and recorded what happened up to the point when the female was mated. In 64% of the cases, the males resolved the dispute about the female by fighting using their large chelicerae. Larger males are better at winning these direct conflicts. But in 36% of cases, males used a different strategy called 'interference'. After losing a fight, they hang around and 'distract' the courting pair trying to chase the winning male away from the web. They actually succeeded and mated the female first in 7 occasions (12% of all conflicts) showing that this strategy - which is adopted by small males - can pay off. Additional observations suggest that smaller males hang around and try and mate with the female (not virgin at this stage) probably winning some residual paternity in the process.
 In a different study, Funke and Huber carried out detailed measurements of males and females genitalia and cheliceae (see figure above) and found out that male chelicerae (including the oversized fangs) grow faster with body size than genitalia, indicating strong directional natural selection on this trait due to the large effect on male-male conflict in their fights for females.

More information
S. Funke and B. A. Huber (2005). Allometry of Genitalia and Fighting Structures in Linyphia triangularis (Araneae, Linyphiidae) Journal of Arachnology, 870-872 DOI: 10.1636/S04-16.1

Nielsen, N and Toft, S (1990). Alternative male mating strategies in Linyphia triangularis (Araneae, Linyphiidae). Acta Zoologica Fennica

No comments: