Sunday, April 6, 2003		Nature

The wonder world of the bumblebee
Nutan Shukla

RELATED to the honeybee, but comparatively more hardier, the bumblebee has adapted well to live and survive in the cooler parts of the world, including the Himalayas. The queen honeybee's job is to only lay eggs but if it wants to start a new colony, it needs a swarm of workers, whereas the queen bumblebee does it all alone.

In spring, when the temperature rises, the snow melts and the flowers bloom, the queen bumblebee sets out in search of a suitable place to start a new colony. Since she has to build her own brood cell, lay eggs and sit on them in much the same manner as a bird to hatch her first brood of workers solely by herself, an intricate dwelling structure is out of question. Her nest has to be readymade, and is usually built in the abandoned hole of a vole or fieldmouse, or in a grass tussock.

After the suitable abandoned nest-hole is found, the queen refurbishes it according to its requirements, lining it with moss and grass. Once the interior is done, she constructs a cup from the wax secreted from her abdomen and after filling it with pollen lays about a dozen fertilised eggs in it and seals it with wax. Fertilised eggs produce female workers. After this, the queen builds and stocks a honey container in the nest.

After sometime, the eggs hatch into larvae and the mother feed them on pollen and honey that she pushes through a hole in the cup. Soon larvae pupate and after about 20 days come out as adult female worker bees. Meanwhile, the queen keeps on constructing cups and repeating the whole process, but soon a time comes when the colony becomes little larger and the queen starts devoting more and more time to egg-laying. The number of bees grows very fast and by the end of summer, the nest-hole is filled with several hundred offspring, produced by one and the same female. This is the time when the queen mother lays two special batches of eggs, one of which will produce drones and the other queens. The new bumblebee queens and males leave the nest and fly off to mate and, in the females' case, to find new colonies after hibernating. Of the total number of newborns, only the young, fertilised queens survive to start new colonies in the following spring.

Like many other bees and wasps, bumblebees too are able to reproduce without actually fertilising their eggs. Reproductive females in these insects mate in summer or autumn, and the sperms received are stored in receptacles inside their bodies. The most interesting part is that bumblebees mate only once but the supply of sperms lasts their entire lives - which can be up to little more than a year. During her entire lifetime, the queen exercises total control over her store of sperms and decides whether or when to use them for the purpose of fertilising eggs as they pass through her reproductive system. Each egg is fertilised with one sperm, but if she withholds them the unfertilised eggs produce males.

Belonging to the order of bees and wasps, Hymenoptera, bumblebees are easily recognised by predators, as they too are able to inflict a painful sting like their relatives. Usually people believe that bees and wasps bite if handled, which is not true. Instead it is their sting, a modified ovipositor (egg-laying structure) placed at the end of the body that causes pain when injected. This also makes it clear that only females can cause pain.

Whenever this weapon with a venom sac attached to it is used, it usually remains stuck in the wound, causing a shooting pain and swelling to the victim and the bee dies. It is the non-producing worker bee that stings and lays down her life in the defence of the colony.

As has been already mentioned, these insects live in a cold environment. Naturally, the question arises as to how these tiny creatures survive such harsh conditions? Bumblebees are among those animals that demonstrate substantial variations in body temperature between different parts of their bodies, a phenomenon called regional heterothermy.

Since these insects are smaller in size, they have large surface-to-volume ratios, which plays a major role in losing body heat rapidly. The flight muscles of these insects also operate properly only at temperatures between 30 degrees and 40 degrees C. In such conditions it is very important to keep the body warm to survive, so the bumblebees, like many beetles and moths, have evolved a mechanism for conserving in their thoraxes (part of the body between neck and the abdomen) much of the heat produced by muscles as they go through very rapid contraction cycles, with the result that the thoraxes are much warmer than the rest of their bodies. With this system arises another problem of heat being lost through diffusion and convection to other colder parts of the body.

To counter this problem insects have evolved some more mechanisms, which helps them in solving the problem to a large extent. For one, they have modified cuticular scales on their outer thoracic surfaces that resemble their fur and, secondly, they have the ability to control the pattern of blood flow in their bodies so as to restrict heat transfer from the thorax to other body parts.

This feature was published on March 30, 2003