The female case-bearing leaf beetle Neochlamisus platani tries to give her children a head-start in life, but most mothers might not be keen on how she does it - encasing her young in an armoured shell made of her own faeces. After she lays her eggs, she seals each one in a bell-shaped case. When the larva hatches, it performs some renovations, cutting a hole in the roof and enlarge the structure with their own poo. By sticking its head and legs out, it converts its excremental maisonette into a mobile home, one that it carries around with them until adulthood.
All leaf beetles do this, but N.platani adds extra features to its home. It does some interior work, adding an attic and filling it with hairs (or trichomes) from the host plant, the American sycamore. It also plasters the exterior with these hairs, giving it a fuzzy appearance. The trichomes aren't just for show. Christopher Brown and Daniel Funk from Vanderbilt University have found that they help to deter predators, over and above the protection provided by the faecal case itself.
Brown and Funk praise this "elaborate example of faecal architecture". They point out that faeces are an excellent building material, combining malleability with low production and collection costs. Those of the leaf beetle are also reinforced with undigested plant matter, giving them extra strength. Nonetheless, lugging your home around for most of your childhood takes time and energy, so it must have some benefit.
To find out, Brown and Funk pitted larvae against local three predators - a cricket, a spined soldier bug and a green lynx spider. When inside their faecal shelters, the larvae were less likely to be attacked by the predators than they were when they were denuded of their cases. Unprotected, the larvae fell prey to all three predators. The addition of the trichomes made the larvae particularly resistant to crickets when they were pupating into adults (although soldier bugs weren't so deterred).
The larvae are otherwise poorly protected with little in the way of defensive spines, hairs or plates. Hiding among faeces provides them with a disguise. Brown and Funk think that the protection they provide would be even greater in the wild, when they would appear in natural contexts as opposed to sitting conspicuously in a Petri dish.
Even if a predator investigates the case, they must first breach the unappetising shield, and the larva doesn't make it easy for them. Brown and Funk saw that, in some cases, the larvae pulled their cases down flush with the floor, making them even harder to penetrate. That defence was particularly effective against the bugs, whose stabbing mouthparts couldn't break through the wall of the case. Some of the larvae also wiggled their cases back and forth, which could serve to shake off or startle a predator.
Even if a predator breaks through the case (as frequent holes in the structures suggest), they'd meet a large concentration of trichomes in the attic before they reached the larva underneath. The sycamore's hairs are irritating, and they could also improve the camouflage of the case. At least one other insect, a lacewing, also uses trichomes for defence, sticking them to its own body as a suit of armour against predatory bugs.
Whatever their benefits, it's clear that these cases have been around for a long time. Individuals preserved in amber tell us that leaf beetles have been building homes of out faeces for over 45 million years. How the group evolved to make these structures is a mystery and one that Brown and Funk hope will be examined by studying existing members of the group. As they say, "the present study should only be the start of rigorous investigations on this fanciful faecal trait and the animals that live within."
Reference: Brown, C., & Funk, D. (2009). Antipredatory properties of an animal architecture: how complex faecal cases thwart arthropod attack Animal Behaviour DOI: 10.1016/j.anbehav.2009.10.010
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