Tachinids are flies – insects belonging to the insect order ‘Diptera’ and, within that order, the superfamily ‘Calyptratae’ and family ‘Tachinidae’. Their closest relatives are the ‘house flies’ (Muscidae), ‘blue-bottles’ (Calliphoridae), ‘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.
The vast majority of tachinids attack the larval stages of their host but some emerge from the pupa or (rarely) adult. Eggs are either stabbed 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 hatch and grab the 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. Interestingly, 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 270 species on the current British Isles 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, 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 keys.
The following photos are NOT tachinids but they are the groups that you might mistake for tachinids: