Wasps are renown for their parasitic ways. Different wasp species hunt different insects and spiders, and in a range of different ways.
For example, a female pompilid wasp paralyses a spider then drags it back to her nest, which contains one of her eggs or larvae. The larva progressively consumes the living, but immobilised host as it develops (see Kerri's recent post).
In other cases, the female wasp injects her eggs into the body of a slow-moving target, such as a caterpillar, and allows it to go on its way. The unfortunate host is then eaten from the inside by the developing wasp larva.
But I only recently encountered the logical development of this strategy. Rather than laying your eggs inside a larva, lay them inside another egg!
A couple of weeks ago I saw a Common Brown butterfly (Heteronympha merope) showing the characteristic behaviour associated with egg laying - fluttering over the ground, landing then arching her abdomen forward. I watched closely as I wanted to collect some eggs to monitor the embryonic development of this species.
Eventually, I discovered a pair of eggs on the leaf of a Flatweed (Hypochaeris radicata) in the area where the female had been laying.
I've since been able to confirm that these are indeed the eggs of Heteronympha merope, although I can't be sure how much time had passed between egg laying and my collection.
Over the following days, I examined these eggs under the microscope and tried to image the developing embryo inside.
Days 4 to 8
By day 4, I could see an elongate white form inside the egg but this showed few features apart from some small bumps. A strange feature was a mass of yolk on one side of the egg. There was little change evident over the following 4 days.
Days 10 to 12
Then on day 10, I saw something totally unexpected - each embryo showed a pair of prominent orange compound eyes and a group of three simple eyes (ocelli) between them. These became more strongly pigmented over the next two days. In addition, the embryos appeared to be developing long antennae.
This was all very puzzling because caterpillars don't possess compound eyes, and their antennae are quite short. What was going on here?
Days 13 to 16
By day 13, a dramatic change had taken place in my original pair of "butterfly" eggs. The creatures inside - whatever they were - were now darkly pigmented and their antennae were much more distinct. Limbs were also taking shape and wings could be seen. These changes progressed further over the next few days. Twitching of the limbs was apparent by day 16.
By day 18 (today!), twitching of the legs had stopped. I decided to dissect away the egg shell (chorion) to release the creatures within. I needed to know what they were!
To my surprise they began moving actively when released from the chorion. One of them stood up, wandered away and proceeded to preen itself. It was now very clear that this was indeed a wasp. But an extremely tiny wasp - only 3/4mm long!
It's a wasp - but what sort of wasp?
A bit of research revealed its identity - the parasitic wasp Telenomus, which belongs to the family Scelionidae. Both of my specimens turn out to be males.
The whole process
Knowing that my butterfly eggs were parasitised by a wasp, I can begin to understand the changes I was seeing.
From days 1-9, the wasp embryo develops into a larva. In the case of Telemonus, this is a very simple animal - a simple tube lacking legs. The bumps I saw were probably the larval mouthparts. The larva feeds on the yolk inside the butterfly egg - the butterfly embryo doesn't stand a chance! It probably didn't survive for more than a day or so. All that is left of it is the streak of yolk seen on one side of the egg from day 4 onwards.
By day 10, the wasp larva is ready to pupate. Over the following 9 days it rebuilds itself, developing the complex organs and body parts of an adult wasp.
But how did the wasp get inside the butterfly egg in the first place? Quite simple really. The female wasp seeks out a candidate egg, thrusts her ovipositor through the chorion, injects one or more eggs, then withdraws the ovipositor.
Some obvious problems
- The hole made by the ovipositor must be small to avoid bursting the host egg.
- The wasp eggs must be correspondingly small.
- Egg parasitoid wasps are small animals and their ovipositors are correspondingly small and needle-sharp.
- The developing wasp obtains all of its nourishment from the host egg, so it can be virtually yolk-free – and tiny!
And my wasp species is not alone in using such a strategy. It turns out that such parasitism of the eggs of other insects or spiders by wasps is widespread. Sixteen families of Hymenoptera can do this trick, and 3 families - Scelionidae, Myrmaridae and Trichogrammatidae - are exclusively egg parasitoids.
I haven't yet witnessed a parasitic wasp injecting an egg with its own eggs, let alone imaged that process. However, here is a link to a very nice video which shows how this occurs in a different species of parasitic wasp (Trichogramma, family Trichogrammatidae). The video is rather long - about 13mins – but I enjoyed every minute!