Azure Damselflies ovipositing

On Monday, I went to my local wildlife garden to check on dragonflies and damselflies. There were only Azure damselflies, Coenagrion puella. The Azure Damselfly is a common species, frequent in small ponds. Males are blue and black and females greenish and black. It can be distinguished from the Common Blue Damselfly by a forward pointing sput on the sides of the thorax. I walked around the pond and counted three mating pairs in tandem, and many individuals, including two teneral ones - just emerged and still not showing their full colour. The females were egg laying, with the males clinging to their thorax, guarding them as they lay from the attentions of other males. The females insert their eggs into stems of submerged aquatic plants. Females can lay over 4,000 eggs in their lifetime. I noticed that the pairs were not randomly assorted around the pond, but tended to gather in the same spot (top shot). Indeed, ingenious field experiments by A Martens showed that this species form aggregations to oviposit, the pairs particularly attracted to the vertical posture of the male in a tandem. Pinned models of a male in vertical position attracted tandems, but pinned models of females did not. Two model tandems were more attractive to pairs than single tandems, which will start an aggregation.

An adult male resting.

A teneral male, recently emerged.

Two pairs egg laying. 

The exuviae of three damselflies still stuck to the stem of a plant. 

More information

Martens, A. Field experiments on aggregation behaviour and oviposition in Coenagrion puella (L.)(Zygoptera: Coenagrionidae). Advances in odonatology 6, 49–58 (1994).

Pair of red Mason Bees

While I was gardening, a furry, buzzing ball drops from the sky into the bottom of a clay pot full of broken crocks. I peek in: it is a pair of Red Mason Bees and the female, with the male firmly holding onto her back, has trouble climbing out. I give them a helping hand and place them on a dandelion. The female quickly starts to feed, greedily on the nectar. The male keeps holding on. He vibrates his wings making an audible buzz, drums with his antennae, kicks her abdomen and pushes with his head against the female's head. I know that Red Mason Bees do mate guarding, a behaviour by which the male 'piggy-backs' onto the female after mating to prevent her to be inseminated by a different male, allowing his sperm valuable time to fertilise the female's eggs. I have no way of knowing if they had already mated at the bottom of the pot, but the male's behaviour was far from a passive 'guarding': he was very busy indeed and oblivious to the food nearby. He carried on for a few more minutes and then flew away. The female fed for a while longer.
 You can watch a short clip here:

Courting spiders

I have posted on this common garden species, Linyphia triangularis courtship before, but I had the chance of observing this pair of spiders courting in the garden yesterday and take some photos. The male is larger, reddish, with an elongated abdomen and very large chelicerae. The female, at the back, has a rounded abdomen with a black and white pattern. As you can see in the bottom photo, there is a recent moult hanging from the web - I presume is the female's, as I cannot see the large chelicers of the male. In this species, the male does a form of mate guarding, sharing the female's web to wait until she has moulted and therefore she is ready for mating, and fending other approaching males off. The male vibrated its pedipalps and abdomen and slowly approached the female.

Water slater embrace

Our pond does not even qualify as a pond. It is half a wooden barrel, almost full of dead leaves and overgrown irises and marsh marigolds. Still, it's got some water snails and frogs have bred the last couple of years. A few days ago, I was watching the very active ramshorn snails when I noticed this paired water slaters, Asellus aquaticus. Water slaters are isopods, like woodlice, but unlike their terrestrial relatives they display a behaviour which is common in crustaceans, mate guarding, by which males spend some time holding onto the female before the actual copulation occurs. This mate guarding, or pre-copula, is thought to have evolved in response to a very short fertile time window in females. As in many crustaceans, females can only mate right after moulting, so this guarding behaviour provides males with the reward of a fertile female at the end, and is more effective than searching randomly for receptive females.
 Unlike most sexually dimorphic crustaceans, male water slaters are larger than females. This is thought to have evolved by sexual selection. Larger males are more efficient dislodging other males already guarding females, or avoiding being taken over by other males.  Females seem to offer little resistance to being guarded, so sexual selection by females can be discounted. However larger males also pair faster, suggesting they might be better at finding females, and experimental manipulation of antenna length has shown that it is their longer antennae which increases the rate of encounters of larger males with females.
 Mate guarding has also been suggested to have an antipredatory function in water slaters. Water slaters are often predated by newts, and research by Paul Verrell indicated that paired males had a lower predation risk than both unpaired males and females in laboratory trials. Female predation rates are unaffected by being paired (see Table 1 below). According to Verrell two factors explained these results, first, males are much more active when unpaired, as they move about searching for females, so they might be much more exposed to predation. Second, when they are paired, after an attack males have a quick escape response from the female which might reduce their predation risk.

(from Verrell, 1985)


References
Bertin, A. & Cezilly, F. (2003). Sexual selection, antennae length and the mating advantage of large males in Asellus aquaticus Journal of Evolutionary Biology, 16, 698-707 DOI: 10.1046/j.1420-9101.2003.00565.x
Verrell, P. (1985). Predation and the Evolution of Precopula in the Isopod Asellus aquaticus Behaviour, 95, 198-202 DOI: 10.1163/156853985X00127