The Spooky Looking Glass Worm
By Majid Ali
from Ryedale Reporter Magazine, Ryedale Aquarist Society, Yorkshire, England
Surprisingly, glass worms (Chaoborus crystallinus) are not worms at all, they
are white mosquito larvae (phantom midge larvae), and the fly they transform into is a
so-called non-biting insect (gnat).
In my area glass worms are most readily available, through my local pet shop, in the
winter time. The larvae are transparent in appearance giving a spooky look. They are
usually half an inch in size and have two black dots in the body (which can be seen).
Surprisingly these black dots are actually air sacs that work like a swim bladder.
Glass worms usually swim in a horizontal manner and the air sacs inflate and deflate to
move up and down in the water. The larvae breathe through the end of their abdomen and
have two eyes at the front of the body, with a claw protruding as if ready to grab
something or hook onto it. The claws can be used to grab smaller creatures (especially
small fry), so feed cautiously if very small fish or fry are present in your aquarium. The
claws are actually hinged antennae which can be snapped together, thus giving the
appearance of claws.
The size of a glass worm can vary from half an inch to almost two inches in length
(frozen and live). Your fish should love these easily digestible 'see-through' worms
because of the chase and kill or chase and swallow opportunities they will get.
I don't know the exact timing as to when the flies mate in the U.K. but in other
temperate areas adult flies emerge and mate in June/July. When the larvae pupate and grow
into the mosquitoes the females lay eggs in jelly-like discs on the surface of the water.
The larvae then hatch out and start wriggling about in the water.
Glass worms can be found in ponds hanging horizontally (usually) but you must look very
carefully because, due to their transparent appearance, they are difficult to spot, and it
is usually movement that makes them appear more visible.
There are two opinions of the hatching process and development stage of the larvae:
1. Because the glass worms are the young stage of the fly, it won't take long before
they change into pupae and complete their life cycle by transforming themselves into
flying mosquitoes. This transformation process can take up to three weeks, so the amount
of time they will be available for fish food is limited by the timing of their life cycle.
2. But according to the Offwell Wildlife Trust (Devon), after mating the flies (which
live for about ten days) lay eggs as soon as possible and these eggs quickly hatch. Larval
development will take about a year so the larvae (glass worms) will be around all winter.
As I am able to obtain glass worms in the winter time, via the pet shop, I would be in
favour of the Trust's opinion and perhaps option 1. applies to larvae in other temperate
areas? While studying at Shipley College in Bradford I recall meeting an American
gentleman called 'Todd', in the college library. We became friends, and in conversation he
mentioned that he had seen glass worms in ponds. He also told me how his father-in-law had
a preference for feeding glass worms to his fish, how he kept these worms in a
refrigerator, believed the larvae to be hardy and had known them to recover from being
frozen for short periods.
Personally I am not sure if glass worms in the U.K. could survive in frozen ponds but
perhaps this could be the case in other geographical areas?
The local pet shop owner told me that his previous live food supplier had informed him
1. Glass worms can survive in water that has not been treated with de-chlorinator. So
if you place them in water not treated for chlorine they will still survive.
2. He had read an article in which brine shrimps and glass worms were laboratory
examined to see which had higher protein levels - glass worms won.
3. Despite appearing empty and transparent glass worms helped to stimulate colours in
live stock to which they were fed.
4. Glass worms can live for a couple of weeks or more in coldwater. This would be
useful to we aquarists as it would allow us to keep our stock of glass worms alive a
little longer for extended feeding to our fish. Of course we would need to make regular
water changes to any container housing glass worms either indoors or in a fridge.
If I had met this supplier I would have asked the question 'Why are glass worms only
available during the winter time and not on a regular basis?'
To answer my own question I assume the following possibilities:
a. Glass worms are imported into the U.K. from other locations thus their life cycle
timings are different?
b. British glass worms may be smaller in winter thus not pose the same predatory threat
when fed to small fish as they would at a larger summer size.
c. Suppliers may turn to glass worms at a time of the year when other regular live
foods, e.g. daphnia, are unavailable. Frozen glass worms, sold as 'white mosquito larvae'
are relished by my fish just as much as the live form.
Finally, I have heard mixed views about glass worms, e.g. some aquarists say they are
nutritious whilst others believe this not to be the case. Regardless of these opinions if
your fish or other livestock love glass worms then keep on feeding them. As responsible
aquarists you should give your fish a well-balanced and varied diet.