Almost mammals

[Carmen Elsa Irarragorri Wyland]

Almost mammals

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A female ball blattodean (Perisphaerus semilunatus) from northern Cambodia begins to unfurl to reveal long, powerful legs.

I was rummaging one day through the leaf litter on the forest floor in northern Cambodia, looking for insects, when a small, perfectly round ball rolled from a leaf above, bounced off my head, and landed on the ground in front of me. I picked it up to have a closer look, not sure if the object was an animal or a plant. It was about the size of a pea, but black, and very hard. It was an animal, as betrayed by the clearly visible segmentation of its body, but several groups (crustaceans, millipedes, and armadillos, to name a few) use a very similar tactic, and I was not sure which one I was holding (I quickly ruled out armadillos.) After a few seconds a pair of big eyes with two short antennae between them cautiously peeked from a crack that opened on the mysterious sphere. It was a blattodean, but one I had never seen before. Later I identified it as the ball blattodean (Perisphaerus), an interesting animal that experiments revealed to be, thanks to its tight armor, virtually impervious to attacks by ants and other small predators. In fact, the combination of the hard cuticle that forms its exoskeleton with its powerful muscles makes it impossible to unroll the animal without damaging it.

Ball blattodeans of the genus Perisphaerus and several related genera are probably the only organisms other than mammals that exhibit suckling behavior. Young nymphs of these insects have long, almost proboscis-like mouthparts that allow them to access a series of special “mammary” glands on their mother’s underside and suck nutritious fluids.

Rolling your body into a tight, hard ball is a neat trick, perfected by only a few other insects, but there is something else about Perisphaerus that makes it unique among not only insects, but also almost all other animals. Blattodeans, a large, ancient lineage, represented by nearly 5,000 species, is a truly fascinating example of the evolution of parental care and social behavior. Within insects, where good parenting usually amounts to not eating your young, blattodeans display levels of devotion and parental sophistication otherwise found only in birds and mammals. Dr. Louis M. Roth, the late Harvard entomologist who during his long and productive life uncovered many secrets of blattodean biology, was the first to realize the unusual nature of Perisphaerus. While studying these insects he noticed that females were often accompanied by nymphs clinging to their legs, and some of these youngsters had their heads stuck to the underside of their mother’s body. Careful examination revealed something strange – the mouthparts of the nymphs were very long, almost proboscis-like, a trait unknown in blattodeans, whose mouthparts are of a simple, biting type. Looking carefully at the female Roth also noticed that between the bases of her legs were small, glandular openings, and that’s where the young ones were sticking their heads. Could it be that the mother were actually suckling her young? Up to that point only mammals were known to display this type of behavior, but suddenly it appeared that a similar one might have evolved at least one more time in the history of the animal kingdom. The evidence for this is still largely circumstantial, but what we know about blattodeans certainly supports such a possibility. Many species of these insects give birth to live young, and in a few cases the female feeds them until they are ready to start foraging on their own. In the case of the Pacific blattodean (Diploptera punctata) the female develops an equivalent of the mammalian placenta, and feeds the embryos growing inside her abdomen with a rich mix of proteins, lipids, and carbohydrates. But a female with “mammary glands” and nymphs with sucking mouthparts take the maternal care among blattodeans to a completely new level.

North American forest blattodean (Parcoblatta penssylvanica) carrying an ootheca – a hard, nearly indestructible purse that protects the eggs from predators, parasitoids, and desiccation.

A couple of years after my first encounter with Perisphaerus I found myself, in the middle of the night, following through a bamboo thicket a group of fanatical herpetologists who were intent on catching a particularly elusive, possibly new to science frog. We were in New Britain, a large island that is a part of Papua New Guinea, and I knew that I had a good chance to run acrossPerisphaerus again. Sure enough, the very moment I heard a triumphant scream that announced the capture of the unfortunate amphibian, I saw the mysterious blattodean scurrying around my feet. And it was a pregnant female. She gave birth to about 10 young a few days later, and during the two weeks when I kept her in a small container the nymphs always stayed with her, hidden under her body, their mouthparts firmly between her legs. Every now and then she would have a bite of a fruit, but the young ones never left her side or fed independently. And yet they grew. I separated a couple of nymphs from their mother and offered them the same conditions and food I was providing her with – they were dead within three days, while their siblings continued to thrive. This short observation convinced me that the female feeds her young with something secreted by her body, and that they completely depended on it, just like mammalian offspring does. My admiration for insects ratcheted up yet another notch.

Table Mountain blattodean (Aptera fusca) from South Africa is a species that exhibits an extended maternal care.

Of course, not all blattodeans display the same degree of nurturing and maternal sacrifice, but there is not a single species in this group that does not at least try to give its children a safe start in life. The least the female blattodean can, and most do for their eggs is to encase them in a hard, chitinous purse that protects the eggs not only from physical injuries and desiccation, but also creates a very effective barrier to predators and parasitoids. Usually such a container, known as the ootheca, is carried by the female until the eggs are almost ready to hatch. She will then burry or glue it close to the source of food, usually a fruit or some particularly tasty leaf, and the young hatch a few days or weeks later, ready to start independent life. In more advanced species the female never lets her eggs go, and while she still protects them in an ootheca, she carries it until the very day the young ones are going to hatch. Others take it a step further and, after forming the ootheca and filling it with eggs, suck it back into their abdomen. There, protected by both the ootheca and their mother’s belly the young ones complete their development. Their hatching takes place inside the mother’s abdomen, giving the impression of live birth (such false live birth is known as ovoviviparity.) And finally, there are species, such as the Pacific Diploptera punctata that are truly live bearing.

A large, shield-like pronotum protects the head and front legs of the giant blattodean (Blaberus giganteus) from Guyana. This species is ovoviviparous, which means that eggs are carried by the mother until they are ready to hatch and the nymphs are ready to start independent life.

I have always been fascinated by these animals: the simple elegance of their bodies, their devotion as parents, their dominance in tropical ecosystems, their ancient origin, all this made me want to learn more. But for a group so rich in species and so abundant in many terrestrial ecosystems, we know shockingly little about blattodeans. There are probably no more than 20-30 scientists worldwide who study the 5 thousand or so species we already know about (an equal number of new species of blattodeans most likely still awaits discovery.) At the same time thousands of students and researchers around the world work on mammals, a group with a comparable number of species. As it turns out, the two may have a number of astonishing similarities in their reproductive behavior. Perhaps some of the mammal specialists could be enticed to broaden their taxonomic horizons, and help us learn more about one of the most intriguing groups of animals that ever walked the Earth? Entomologists could really use some help here.

Wood blattodeans (Cryptocercus) took their family life to the next level, and these insects live in small, multi-generational societies. Females feed their offspring symbiotic protozoans, which these insects need to be able to digest cellulose, their main source of food. From here it took only a small evolutionary step towards eusociality, which we see in a lineage of blattodeans known as termites.