Written by Joe Ballenger
We get a lot of identification requests on our Facebook page, and we do what we can to identify them. Sometimes, these turn into interesting conversations…which in turn lead to good questions like this:
The moth in the picture above is most likely a Fall Cankerworm moth, Asophila pometaria. These moths are interesting because they’ve made an evolutionary trade-off which is uncommon in moths: the females have traded their wings for the ability to make more eggs. It’s a simple resource allocation trade-off: the larvae of this species are able to fly where they need to be. They use silk to get a lift on the wind, and they usually end up in the proper place. The females simply don’t need to spend the resources building wings.
Because the females can’t fly, they tend to wrap their eggs around sticks of their host trees as seen in the picture in this link. Sometimes, however, the females end up on a flat surface and they’ll make due with what they have…and they’ll lay egg clusters of various shapes and sizes and kind of hang out on top. Typically, they’re hexagonal but they can also be laid in long and narrow strips. It’s a ultimately a question of packing efficiency.
Many other insects, however, do lay eggs in specific shapes. Mantids-the insect group we’re focusing on this month-are an excellent example of insects which have specific egg laying patterns.
So why do mantids lay their eggs in specific patterns?
Mantis eggs are an interesting case, because their patterns are species-specific. If you’re paying close attention, you can identify mantis species by the shape of their egg cases. Here, take a look:
These are the egg cases belonging to the two mantis species most commonly encountered by people. The Chinese mantis on the left typically lays it’s eggs in a somewhat spherical case, whereas the Carolina mantis on the right tends to lay it’s eggs in an elongated case. The reason for these differences isn’t likely to be adaptive, but mantis cases in general are pretty recognizable.
When they’re born, insects usually don’t have a lot of control over where they’re put. The mother chooses a location which looks good, sets her eggs down, and usually abandons them. Insect eggs are more or less defenseless, and there are a lot of predators which eat insect eggs.
There are a lot of ways insects eggs defend themselves. Ladybug eggs, for example, incorporate toxic chemicals into their eggs. There’s also some evidence that some ladybugs use parasites to defend themselves. Mantids use a form of camoflauge. Their egg cases resemble galls, which are growths on plants produced by insects…usually flies or wasps.
To complete the illusion, they’re covered in a styrafoam-like protein covering which prevents them from drying out. The pattern and covering confuses predators which might eat them. As mentioned in Do Mantids get Parasites, their egg parasites fall into one of two categories: they either hitch-hike on the females to find the eggs, or they’re related to insects which parasitize galls.
Insects which have similar problems sometimes sometimes solve them in similar ways, a phenomenon known as convergent evolution. Stick insects, for example, are not related to mantids (Nancy will explain this later) but have evolved a similar body plan which relies on camouflage to solve their problems. For mantids this is mainly an offensive strategy which allows them to sneak up on prey, but both mantids and stick insects use their body plan to hide from predators.
Stick insects tend to lay single eggs which resemble seeds, which is a form of camouflage. However, one group of stick insects has also solved their predator/parasite problem using a tactic similar to mantises…they’ve evolved to lay their eggs in cases in which is very similar to what mantids do.
Insects distribute their eggs in time and space to hide them from predators. Many times, these take shapes which allow females to efficiently cram them in one small area. Other times, the female insects hide their eggs by disguising them as something that potential predators wouldn’t recognize as a tasty meal.
Goldberg J., Jerome Constant, Bruno Kneubühler, Fanny Leubner, Peter Michalik & Sven Bradler (2015). Extreme convergence in egg-laying strategy across insect orders, Scientific Reports, 5 7825. DOI: http://dx.doi.org/10.1038/srep07825