Tachinids are flies - insects belonging to the insect order 'diptera' and,
within that order, the superfamily 'Calyptrae' and family 'Tachinidae'.
Their closest relatives are the house flies (Muscidae), 'blue-bottles'
(Calliphopidae), 'flesh flies' (Sarcophagidae) and some smaller families
- the Rhinophoridae and Oestridae.
Tachinids are commonly refered to as ’Parasitic Flies’ because
the larvae feed on the body tissues of immature or adult invertebrates.
However, although we use the term parasite they are really parasitoids
- the difference being that parasites (like tapeworms) don’t kill
their hosts, but parasitoids usually cause the death of the host in some
way - either by killing them outright - or by weakening them so much that
they die - or in the case of 'clepto-parasitoids', by eating the host's
The vast majority of species attack the larval stages of their host -
but a few have been known to start their attack at the egg, pupa or adult
stages. Eggs are either: injected into the host with a sharp , dagger-like
piercing organ (flies don't actually have an ovipositor, like some wasps);
or laid on the skin of the host (often in places that are impossible for
the host to reach and clean off); or they can even be laid in bulk on
the host's food and can either jump onto a host as it walks past or enter
the host's body through its gut when the egg is eaten.
Most species attack the larvae of butterflies & moths (Lepidoptera)
but some groups specialise in attacking beetles (Coleoptera), true bugs
and plant hoppers (Hemiptera), sawflies (Hymenoptera), and other orders.
Interestngly, very few attack other flies.
Some species of tachinid specialise in attacking a single species of
insect (e.g. Cadurciella tritaeniata,
which attacks only Green Hairstreak butterfly
(Callophrys rubi)) but most seem to
have quite broad tastes. Typically, tachinids hunt for related hosts,
matching a certain size and appearance; within a defined habitat; and
a niche within that habitat. So, for instance: "large, hairy moth
caterpillars that usually sit exposed on low-growing heathland vegatation"
(a favourite of Tachina grossa, our biggest tachinid) OR "medium/large
butterfly & moth caterpillars that usually sit under the leaves of
deciduous trees along woodland edges".
We have over 260 species on the current UK list and this number is constantly
under review as new species are being found each year. Most newcomers
are species genuinely expanding their range from mainland Europe or which
are very local in distribution or which are seldom encountered because
you need to use unusual trapping methods (eg. Malaise Traps) to catch
them. However, we have gained some due to taxonomic revisions (where existing
species are redefined and split into 2 or more species) and a few through
closer examination of museum material using European, rather than British
If all this has whetted your appetite for more then please read on and
see the studying tachinids page