The cost of long-horned moths long antennae?

As I walked past the university woodland area, in the usual clearing, a swarm of 7-8 Yellow-barred Long-horn moths, Nemophora degeerella in their bobbing flight. A few other males sat on the leaves facing the swarm. I searched for females to no avail, but something caught my eye, a male that had been caught in a spider web by his antennae (top shot), still alive, but kicking hopelessly. I have covered these moths in previous posts, the extraordinary antennae of their names only applies to males: the females have a much shorter antennae. This sexual dimorphism suggests that the male's antennae have evolved in response to sexual selection, possibly in relation to pheromone detection by males.

Two males rest on leaves (1/06/2017).

The evolution of exaggerated sexually-selected traits often is accompanied by costs and in the case of these moths, increased predation risk is a likely prize the males are paying for their oversized antennae.

A large swarm of long horned moths on 1/06/17.

Cybaeidae: water spiders

Credit: Norbert Schuller CC BY-SA 3.0 via Wikimedia Commons

The only British species in this family, the Water Spider Argyroneta aquatica, is the only known spider that lives almost permanently under the water surface. It inhabits clear ponds, ditches or slow moving streams. Despite their ability to dive and swim freely, their adaptations for aquatic life are subtle: the fine hairs of their abdomen and front of cephalothorax trap a silvery lining of air when the spider is under water, and the two pairs of rear legs are covered with rows of long fine hairs, which help with swimming and collecting air bubbles. They are grey and brownish spiders that can reach large sizes (sometimes over 2 cm).

Inside diving bells
The water spider builds an underwater refuge made of a dome of silk attached to submerged plants which she fills with air. This takes several trips to the surface, trapping large air bubbles with the aid of her rear legs, and then releasing the air out when she is under the dome. The dome then acquires a bell shape and effectively acts as an external gill. The spider spends much of its time in the bell, as oxygen diffuses from the water into the air bubble as the spider uses it up. Moulting either takes place inside the bell or on the water surface.

Reverse size dimorphism
Unlike most spiders, water spider males are on average larger than females. This could be due to the different hunting strategies of males and females: males are more active hunters and spend more time diving outside their bells, and large males are better divers than small ones in moving inside water as they are better counteracting air buoyancy. Females build larger bells and, in contrast to males, tend to practice ambush hunting from their bells, sitting head down with her front legs by the entrance, where they can detect vibrations of passing prey. Both males and females take their prey to their bells to eat.

Reproduction

Mature males (top shot) leave their bells in search for females. Mating takes place in the female's bell, although it involves both male and female swimming outside the bell. When mating, females prefer larger males, but they are also more wary of them, as - also uniquely to them - male water spiders are known to occasionally cannibalise females (although the reverse also happens). Females sometimes lose their bells after rejecting a male.
The female lays her eggs in her bell, in a thick white egg sac at the top of the bell. She partitions this area out with silk and remains in the bottom part of the bell herself. She stays there on guard until the spiderlings hatch. After a few moults they disperse and they quickly build their own little bells under water. Females can lay several egg clutches in a season.

Empty snail shells
At the end of autumn, the spider reinforces the bell with more silk, closes it and stays in it through the winter months. An alternative winter home are empty pond or ramshorn snails, lined with silk and filled with an air bubble.

More information
Schütz, D., & Taborsky, M. (2003). Adaptations to an aquatic life may be responsible for the reversed sexual size dimorphism in the water spider, Argyroneta aquatica. Evolutionary ecology research, 5, 105-117.

Schütz, D., & Taborsky, M. (2005). Mate choice and sexual conflict in the size dimorphic water spider Argyroneta aquatica (Araneae, Argyronetidae). Journal of Arachnology, 33, 767-775.

The weird and wonderful Vapourer Moth

While having lunch, a bright, russet moth outside flew against the bay window glass again and again. After a while, I decided to capture it to check what it was. The moth was very unsettled, and carried on flying in the bug pot while we finished our meal. Shortly after, another similarly russet moth, a bit larger, flew against the glass and landed on a silky cocoon on the window frame. I went out and realised the reddish moths were in fact male vapoureurs attracted by a female on the cocoon. The second male was now mating with the female (above). The Vapourer, Orgyia antiqua is a very odd moth indeed. They have a striking sex dimorphism. Males have large, feathery antennae and often fly during the day. They are a rich russet colour with a white spot on each forewing. Females, in contrast, don't look much like moths, they cannot fly as their wings are vestigial, they have small antennae and enormous bulging bodies, full of eggs, and are pale grey in colour. Females will barely move in their short lives, attracting the males with pheromones they release shortly after emerging from her cocoon. The large feathery antennae of the males helps them locate the females quickly.

I took this photo from inside the house.

After mating for about 15 minutes, the male left (13:20). We could then had a closer look at the female. Eggs were visible through her thin abdominal skin. She looks velvety and heavy.

The female quickly started to lay eggs on the cocoon surface, the photo below taken at 15:15. Note that the caterpillar hairs, which are irritating, are embedded on the cocoon itself, so they may act as a defensive device for the eggs themselves.

After egg laying, female dies, so the adult stage is very short, about two days and they do not feed. The eggs overwinter and the caterpillars will be born the following spring.
  There are several British species of a few moth families showing this pattern of female flightlessness, amongst them the Winter Moth. The limited mobility of the females is compensated by the highly dispersive larvae, which might be able to balloon when little. In the UK adults are found from July to September.

On this photo you can see the vestigial wing: just a small hairy flap (the head is down and the abdominal tip up, egg laying).

At 16:41 she had pretty much finished laying.

 I searched around for more cocoons nearby and found one under a wooden shelf by a large cotoneaster, about 2 m away from the first one. It looked very fresh and translucent, and still contained a caterpillar.

The large oval cocoon and large caterpillar inside points to another female will emerge from this one too.

More lesser stag beetles

My kids found two live Lesser Stag Beetles on the pavement near the tree stump, now covered on flowers, where they grow. I couldn't resist the photo opportunity. As they often get squished, we took them home for release in our log pile after a few photos. This year we have found 4 females (2 live, 2 squished) and this male. They are my favourite beetle, the males are so powerful looking but still they have nice large smiling eyes. If I tickled him on the head with a small stick, he raised the front of his body and opened his jaws wide (above), awesome!

Male on relaxed position.

Dorsal view

 Female (left) and male (right). This is one photo in which I positioned both individuals opposite each other. Note the differently sized head and mandibles, despite similar length. The surfacing of thorax and head are also different in both sexes, in the male they look matt, while in the female they are punctated and shiny. Both sexes dig soft, rotting wood with their legs. The male still had some wood stuck to his legs.

This view shows the 'tooth' on the jaws.

For more posts on Lesser Stag beetles on BugBlog click here.

The curious Autumn spider mating tactics

Female spiders tend to be larger than males. Large females are thought to be selected for as they can produce more eggs. The relative size differences between males and females depend on the type of competition between males, that is, on the importance of intra-sexual selection. When males do not compete much with one another they are smaller than females, whereas when there are male-male contests for females, then large males are selected for, and size differences with females become smaller. The spider, Metellina segmentata, is an orb spider common during September and October, when most mature individuals are found. Males are more powerfully built and have longer legs than females, suggesting that male-male contests occur in this spider. When young, males build their own webs to hunt insects, but upon reaching their final moult, they mature and start searching for female webs. Once they encounter a receptive female web - likely through detecting a pheromone - they adopt a curious sit and wait strategy. They have to wait, often days, until a large fly falls in the web. Only when the female starts eating the fly does he starts his courtship, taking advantage of the diminished cannibalistic tendencies of sated females. Of course, during this wait, other suitors might arrive to the female's web and when two males encounter each other fights follow. Males are often injured or killed in these fights, where the larger male has an advantage and this is what provides the selective pressure for powerfully built males.
 In addition, males are able to monitor several female webs when they are nearby and this results on males guarding webs becoming larger as the season progresses, and a pool of smaller wandering males.
 The mature female Metellina on the top photo has hung her web on a tomato plant since at least mid september. I haven't noticed any males but I shall keep an eye for these in the next days.

Reference
Prenter, J., Elwood, R., & Montgomery, I. (2003). Mate guarding, competition and variation in size in male orb-web spiders, Metellina segmentata: a field experiment Animal Behaviour, 66 (6), 1053-1058 DOI: 10.1006/anbe.2003.2266