Tachinid flies and Monarch Butterfly caterpillars

In my column (in the Tenerife Sun newspaper) I had recently reported about some Vine Hawk moth caterpillars I had rescued from a garden and was looking forward to seeing the adult moth. Sometimes, however, when rearing insects you end up with something very different to what you expected.

There are countless forms of parasitic flies and wasps that lay their eggs on or in caterpillars but the host larval insect looks as if it is healthy until it reaches the later stage in its metamorphosis. Some parasitic species emerge as grubs when the caterpillar is about to change into a chrysalis and others take the situation one step further and are still inside the body of their insect host in the pupa.

So what you finally end up with is not a pretty butterfly or moth but an ugly parasitic fly or wasp. It always reminds me of the scene in the Sci-fi movie Alien where the extraterrestrial creature breaks out of the body of the astronaut it has invaded.

Fortunately, for me I got used to seeing such parasites emerge from caterpillars I had kept as a small boy but I am sure for many people it must be a real nightmare to see this sort of thing happen. Unfortunately this was to be the fate of all the Vine Hawk larvae and it also happened to some Monarch butterfly caterpillars I found.

So I still haven't seen an adult Vine Hawk but I have seen a fly that is a parasite of this species. It was really disappointing but I released the adult parasites, which in their own way have just as much right to life and have an incredible life cycle even if to our eyes it seems gruesome.

The parasite flies go through all the same stages as a butterfly going from egg to grub to pupa to adult. The difference is that unlike a caterpillar they don't eat leaves but must feed on the living flesh of caterpillars they live inside.