Winter gnats

With the winter being generally mild, there have been small swarms of winter gnats Trichocera sp. in the garden throughout, with their larvae probably feeding on the rotting leaves that we've piled up to make leaf mould.

Lately these long-legged little flies have gravitated towards the last few crab apples that are still hanging on the tree, gently rotting in the spring sunshine.

It's a convenient site for mating. When a female strays into the dancing swarms of males she's quickly captured ..... and so begins the next generation of winter gnats. The male is on the right. 

Nauseatingly beautiful...

The colour of a greenbottle fly Lucilia sericata is undeniably exquisite - an iridescent, metallic shade of green that, if it graced a beetle or maybe a damselfly, would be unreservedly admired.

In sunlight the exoskeleton of the fly almost looks like polished brass. 

Unfortunately there's another side to greenbottles that's rather less attractive but nonetheless fascinating - and if you don't want to know about it, read no further..........

...........but, on the other hand, if you can't contain your curiosity, read on .......

When an animal dies it's often a greenbottle that is the first fly to find the corpse, which it can detect over long distances. The rate of development of the eggs that it lays and the maggots that hatch have been well calibrated in laboratory studies, so forensic entomologists can say with some accuracy how long a body has been dead by looking at the stage of development of the maggots, which take around four days to develop (depending on temperature) before they crawl out of the corpse and pupate in the soil nearby.

Greenbottle maggots feed on dead tissue and bacteria that contaminate it, so they have been used medicinally in maggot therapy, where sterile maggots preferentially eat necrotic tissue around wounds and also secrete a natural antibiotic, speeding up the healing of wounds. I don't know whether this treatment is available on the National Health Service yet but the NHS has certainly taken an interest in it and its use was debated in the House of Commons in 2007.

If you click here you can read a scientific paper that describes the history of maggot therapy and its modern applications. You can also read a Wikipedia article on the subject by clicking here.

Menace

Thursday's Guardian Country Diary describes the mosquitoes that breed in almost every small accumulation of rain water in our garden. The sultry wet weather that we've experienced lately has provided perfect breeding conditions for the larval stages and I've been watching their development.

These are the larvae, hanging from the water surface by their tail breathing siphons. Those jaws are less menacing than they look - the mouth is fringed with small whiskers that constantly move, creating a water current and filtering out food particles. The larvae of this species, Culex pipiens, moult four times as they grow and you can see two developmental stages in the photo above - the larger larvae are at their final stage of development and are ready to become .....

....... pupae. You can see a comma-shaped pupa floating amongst this gang of larvae.

The pupae hang from the surface with two breathing tubes on the head that resemble devilish little horns.

The pupal stage is relatively brief and as the mosquito develops inside and they move closer to hatching they become darker, like the one on the right. The larvae are very active, wriggling down to the bottom of the water at the slightest disturbance, but the pupae are positively hyperactive if you disturb them, tumbling as they dive. 

When they are ready to hatch the pupae float passively on the surface and their tails straighten, like the one at back that you can see here. Then ....................

Their exoskeleton splits along the back and a mosquito's head and antennae appear above the surface.

Some don't make the successful transition from water to air if they are disturbed during this critical step - the one on the left has become trapped in the surface tension and has drowned; you can see the empty pupal skin floating below it. 

The photographs below show several more stages in a successful emergence. Adult mosquitoes look menacing from the moment they appear above the surface, don't they, with those intricate, ultra-sensitive feathery antennae and suctorial mouthparts? They have their own brand of sinister beauty - you can see view some images of their rather beautiful eyes by clicking here.

The sequence of  six photographs below show another successful emergence. 

It's only in the third photograph that the legs begin to play a part; before that the insect just wriggles out of the pupal skin, perhaps aided by those backwards-curved hairs. The strange discontinuity that you can see bisecting the mosquito here is an optical artefact, caused by the water meniscus on the side of the glass container.

Once the insect's legs are free it can rest them on the surface film and finally lever its abdomen out of the pupal skin, until ...........

..........finally it's free. 

The species in these photographs is Culex pipiens. It spends autumn feeding of plant juices and the mated  females that survive the winter will need a blood meal before they can lay eggs and produce the next generation in summer 2014. Fortunately this species feeds on the blood of birds rather than humans.

Many will not survive the autumn; thousands become trapped in spiders' webs. They are also important food items for birds like swallows and house martins and the pipistrelle bats that hunt over our garden will probably catch them in large numbers too.

Natural insecticide

I found numerous examples of mummified, dead flies of various species (but mostly dung flies) attached to plant stems in the garden during the early days of July, although the incidence of diseased flies has declined during the current hot weather. They've been killed by the fungus Entomophthora muscae.

The fungus invades the fly's body and slowly weakens it. In the final stages of infection the disease modifies the host's behaviour so that it crawls to an exposed position at the top of a plant stem and then dies. Soon fungal spores erupt from the insect's body, as you can see in the photo above, and the corpse is perfectly positioned to pass on the infection to other flies in the vicinity.

There have been numerous attempts to culture this fungus so that it could be used as a commercial biological insecticide for reducing house fly populations - with limited success.

Dancing fly

These tiny flies that seem to be laying eggs in the water-filled shell and decaying remains of a garden snail (one at 11 o'clock on the shell rim, the other at 5 o'clock just inside the shell) are Sepsis fulgens. They are about 5mm. long and are said to be ant mimics - their black shiny bodies certainly give that appearance, although I've never come across any evidence for the functional significance of this similarity.

They've emerged in large numbers in our garden during the recent spell of hot, humid weather. In this image you can see the two white halteres just behind the wings, which show that these are true flies (dipterans) and not winged ants, which are hymenopterans with two pairs of wings.

Sepsis has very distinctive behaviour, scuttling over leaves, dancing and waving its wings at other flies.

The similarity with an ant is particularly striking in this side view.

Rhingia campestris

This is the commonest hoverfly in our garden at the moment - Rhingia campestris. Here it's on sweet rocket Hesperis matrionalis. When the insect is at rest its proboscis sticks out in front and looks quite menacing, but when it's actually feeding ..

..... the proboscis hinges downwards and the stretched soft tissue between it and the 'snout' that faces forward acts like a pump for sucking up nectar.

This individual has just withdrawn its proboscis from the flower, giving a better impression of how long it is. It's often assumed that this hoverfly is exclusively a nectar feeder but a study published back in 1989 by John Haslett at the Department of Zoology at Oxford University found substantial amounts of pollen in the guts of females, which need the protein that it contains for the production of their eggs. Male R. campestris were mainly nectar feeders, using the sugars as their energy source.

The books say that R. campestris breeds in dung, something of a contrast to the smell of sweet rocket that it's feeding on here - this is one of the most fragrant plants in the garden, with a strong carnation scent.

Eye-to-eye with a fly

I haven't managed to identify the species that this fly belongs to yet, but it does have beautiful eyes.......

No Hiding Place .....

When you cast a big shadow, there's nowhere to hide on a sunny day ....

Picture-winged fly

I found this female picture-winged fly lurking on a sycamore above a patch of creeping thistle yesterday, so by dint of association and the pattern on the wings I'm guessing that it might be Urophora cardui, which lays its eggs in creeping thistle flower heads. 

That long ovipositor on the tail looks like the right kind of equipment for inserting eggs amongst thistle florets.....

             ..... like these.

Common

Buttercups are amongst the commonest of all wild flowers and tend to be taken for granted because of that, but they represent a vital, reliable resource for wildlife at the bottom of the food chain. All that pollen supplies food for an amazing array of small insects. 

This young earwig, was nibbling buttercup petals....

............ while this unidentified beetle was feeding on the stamens........

.... and this hoverfly was making a meal of the pollen.

Understandably, naturalists tend to be obsessed with rarities, but it's the common stuff that's really important when it comes to maintaining the stability of  ecosystems.

Transformers

Drone fly larvae, commonly known as rat-railed maggots, possess one of the most amazing pieces of equipment found in any insect larva. They live in waterlogged soil or shallow water around the margin of ponds, where the water level can rise and fall all the time - especially in a wet summer like the one we are enduring now. 

The long tail is the insect equivalent of a submarine snorkel, with muscles that allow it to shorten or elongate so that the tip of the tube is always level with the water surface.

At this stage in their life cycle they may not be high on aesthetic appeal but when they metamorphose they transform ........... 

...............into one of these 

A life that starts in stagnant mud becomes a drone fly, hovering in a sunbeam on a sunny afternoon.