Can insects get fat?
The following answers were selected and edited by New Scientist staff. You can add your replies in the comments section below.If, by getting fat, the questioner means obese, the answer is no. All insects undergo some sort of metamorphosis, passing through a larval stages before becoming an adult. The adult, or "imago", stage is relatively short-lived and very often adult insects do not feed at all. Mayflies (of the order Ephemeroptera) and many silk moths (of the family Saturniidae) are some examples. They neither have the time nor the inclination to feed and get fat.Those imagos that can feed are constrained by their inflexible exoskeleton. They have no means to expand this to take on excess fat. Incidentally, carcasses of death's head hawk moths that have been stung to death are often found in beehives, where they have made a vain attempt to feed. Their proboscis is too short to enable them to extract nectar from flowers, but long enough for them to consume honey if the bees in the hive would let them.The larval stages are similarly constrained. To grow they have to moult their exoskeleton periodically. Internal fluid pressure splits the exoskeleton and the insect expands into a new one it has grown underneath, which remains flexible just long enough to accommodate their increased size. They cannot keep doing this indefinitely; each species is limited to a predetermined number of moults. If they find abundant nourishing food, they will go through their moults quickly. If the opposite is the case, or if their food contains limited nourishment they will take a relatively long time to finish growing. Either way, once the moults are completed, they begin metamorphosis into the adult stage. They do not continue to get fatter and fatter and, in fact, the reverse can be true - those larvae unable to find sufficient food may begin metamorphosis early, skipping one or two moults. This produces a normal, if somewhat smaller than average imago.However, insects do store up a great deal of fat at the larval stage. Silk moths are generally large insects. Because they do not feed as adults they must have considerable fat reserves to enable the males to track down a female and mate and, in the case of the females, to produce and lay a large number of eggs, while sustaining their metabolism for a week or more. But this is the normal situation, they are not, in any way, obese.
Terence Hollingworth, Blagnac, France
Insect life cycles do not lend themselves to the concept of obesity as we know it. Most species accumulate food as fast as they can, but once they have enough, they enter their next stage of life, or reproduce and die. There are exceptions, but few can afford to get too fat - anything that interferes with their bodily function prevents reproduction, so what they cannot use, they dump. For example, sap-sucking insects get far too much sugar from plant sap, but instead of becoming uselessly fat, most dump the excess as honeydew or convert it into waxy armour. Using hormones to prevent insects from maturing may make them larger and fatter, but prevents their breeding.Still, healthy insects do accumulate fat. Their internal "fat bodies" are special organs crucial for storage, hormonal control, metabolism, growth, overwintering, fuel for travelling, yolk production for eggs and so on. Accordingly, many insects, such as locusts and termites, though not technically obese, are prized as fat-rich foods. As you may have seen on TV, termite queens of most species accumulate huge fat stores to support their role as virtually continuous egg-laying machines.
Jon Richfield, Somerset West, South Africa
With my colleague James Marden I have described (among other symptoms) infection-associated obesity in a dragonfly species. Infected dragonflies show an inability to metabolise fatty acids in their flight muscles and so build up lipids in their thorax, leading to a 26 per cent increase in thoracic fat content. The suite of symptoms caused by this infection includes decreased flight performance and reproductive success in male dragonflies (Proceedings of the National Academy of Sciences, vol 103, p 18805).
Ruud Schilder, School of Biological Sciences, University of Nebraska, Lincoln, Nebraska, US
