The Belostomatidae is a family of giant water bugs (Order Hemiptera) that has been fairly extensively studied because of the various species' unusual reproductive systems. In short, this is one of the few groups of animals that exhibit paternal care of offspring.


Probably the most well known animals with paternal care is the sea horse, who carries its mate's eggs in a brood pouch until they hatch. Belostomatids are similar because the male also takes care of the eggs, although he does it in two different ways between the two subfamilies, Lethocerinae and Belostomatinae. Above is a giant water bug in the genus Lethocerus. They are sit-and-wait aquatic predators, hanging head down on sticks or reeds underwater. An appendage extending from their abdomens remains above water and allows them to breathe. They are quite large, and much of their prey consists of tadpoles and small fish.


They have a somewhat painful bite, because they have a sharp beak with which they inject a neurotoxin which helps them control their prey. However, if one holds them just behind the head as shown it is safe to pick them up. (I was holding this particular specimen (from Costa Rica) that another student and I were studying; we were doing measurements to look for morphometric differences between males and females, and I had to do all the measuring because he was too afraid to pick one up.)

The Lethocerines are thought to be the more ancestral lineage in the family, based partly on the way they brood their young. Females lay eggs along the top of a stem sticking out of the water, but the eggs will dry out and die without care. The male stays as a sentry on that stem, periodically carrying water up to moisten the eggs and oxygenate them.


The Belostomatines are considered more derived evolutionarily, because the brooding behavior seems to be more efficient. Females lay their eggs directly on the back of the male, who must swim around with them until they hatch to keep them properly oxygenated. The picture at left shows a male with eggs.

Scientists like to study "reverse mating-system" species such as these because it gives us clues about what governs decision-making in animals. In the case of mating behavior, in nearly all animals known, females are choosy about their male mates, who often have elaborate physical features or behavior designed to attract the attention of females (or fight off other males). This is why in many species of birds, the males are more brightly colored than the females. The reverse mating-system species allow us to ask questions like, are females always the choosy ones, because they invest more resources in their gametes (eggs are a lot bigger and fewer than sperm), or is the parent with the largest investment overall the choosy one? Gamete size is an important measure of investment, but time and energy invested in a single mate are important too. In most animal species, a male has the sperm and the time to mate with many females, so he tends not to be choosy. A female not only has fewer gametes but usually invests more time in rearing the offspring than the male, so it's more important that she choose a mate with good qualities (for that species).

It has been found that in reverse mating-system species, the males actually tend to be the choosy ones - so there's nothing about being female per se that makes one choosy. Parental care of offspring is a huge investment, so when it switches to the males, they become the choosy ones. In the case of giant water bugs, a female may have enough eggs to mate with several males, and as soon as she lays them she can move on and find another mate. The male is the parent stuck taking care of the eggs for a couple weeks and thus loses opportunities for more matings in that time. Thus, accordingly, belostomatids and sea horses tend to have choosy males rather than females.

Labels: cool bugs, ecology, evolution, insects, sex