Extreme Insects. Richard Jones

by molluscs and vertebrates. Snails and slugs use it to lubricate their path as they glide forwards on their own moist layer, and to a certain extent as a defence, since the stickiness deters predators, which can get gummed up in it. Vertebrates use it to line their airways, guts and genital tract, and to cover their eyes, where it forms a gel layer in which antiseptic enzymes can protect against microbial attack. Mucus is a very sticky substance, and very useful, so it will come as no surprise to learn that some insects use it after all.

      Mucus is made up of mucin molecules – a number of long protein chains covered with atomic groups which resemble sugar molecules. The sugar parts (glycans) attract water (and each other) and as the long mucin molecules slide past one another, these areas act like weak glue, partly sticking the strands together. The mucus remains wet and tacky, and does not set hard like that other important long-chain protein molecule, silk, which is produced from the salivary glands of many insect larvae, which use it to spin a cocoon in which to become adult.

      Fungus gnat larvae produce mucus from their salivary glands, but they do this throughout their larvahood, not just during metamorphosis at the end. The larvae of these small midge-like flies live under dead logs, fungal fruiting bodies or in caves. Here they build a rough sheet web of sticky mucus strands, covered all over in tiny water droplets. Sometimes they add a soft flexible tube into which they retreat for shelter. Many species eat highly nutritious fungal spores. The spores are impossible to catch when airborne but are caught in the gleaming mucus, and can then be eaten. The webs of some species also contain oxalic acid, a simple chemical similar to vinegar but much more powerful. It is highly toxic to many animals (including humans), and the gnat larvae use it to kill insect prey, which they then eat too.

Biggest blockhead

NAME	carpenter ants Colobopsis truncatus (and other species)
LOCATION	worldwide
ABILITY	uses its head as a living gate at the entrance to its burrow

      Ants gain protection from a complex social hierarchy that generates workers to forage and build, and soldiers to fight and protect. The nest that they build and protect is the ants’ most important asset. Ants need to protect their nest from many enemies, including predators, parasites and other ants who would like to raid the valuable protein invested in the brood as well as any food stores laid up against hard times.

      Soldier carpenter ants have evolved huge mallet-shaped heads with which to bar their nest entrances. Small holes are blocked by a single soldier, while for larger entrances several soldiers gather together to form a living barricade. The soldiers seldom leave the nest, but are fed by the workers that constantly come and go.

      When a worker needs to exit or enter the nest (see opposite), it is recognised by the blocking soldier, which pulls back into the broader tunnel behind. It is thought a combination of the host nest’s chemical smells and the ‘right’ tactile signals from the worker’s antennae identify it as a fellow citizen. If there is an attack on the colony, alerted ants release a chemical called undecane from a gland in their abdomen. This creates rapid excitement of other ants, and the many soldiers rush to block all external and internal tunnels.

Most sexually dimorphic insect

NAME	European snail beetle Drilus flavescens
LOCATION	mainland Europe and the UK
ATTRIBUTE	most extreme difference between male and female

      Males and females are different. Males produce huge amounts of tiny sperm, which they generally try to spread about between as many females as they can. Females carry the eggs, and although they may benefit from males competing for their attentions, multiple matings carry a cost in terms of time wasted and sometimes even physical damage. These different biological drives often produce very different behaviours in male and female of the same species, and sometimes also different body forms. In most insects these structural differences are small, but in one group of beetles, males and females are so different that they look like completely different organisms.

      The European snail beetle, Drilus flavescens, is small (4 to 7 mm) and brown; it has a black head and thorax, and feathery antennae – at least the male has. The female, by extreme contrast, is a large, soft, flabby, caterpillar-like creature, 50 times as large as the male. The males fly on hot sunny days, but the females lack both the normal hard beetle wingcases and also the functional membranous flight wings. The distribution of the males shows that the species is fairly widespread on limestone or chalk soils, but despite this the female is virtually unknown. In fact, the female of this peculiar species is so rarely seen that there was no reliable published picture of her until this mating pair was photographed in 2003.

      The larvae of Drilus eat small snails. Despite being a widespread insect, the rarity (or perhaps the secretiveness) of the females and larvae meant that the beetle’s life cycle was not worked out until 1903. Quite why males and females of Drilus should be so very different is still a bit of a mystery, although many female glow-worms (also beetles but in a completely different family) are also wingless, and their larvae, too, are snail predators.

Most mixed-up sexuality

NAME	bilateral gynandromorph various species, but particularly prominent in butterflies
LOCATION	this example was bred in captivity
ATTRIBUTE	half male and half female

      Insects are usually either wholly male or wholly female. In extremely rare situations, however, there appears an individual that is exactly half one sex and half the other – a bilateral gynandromorph – and nowhere is this more striking than when it involves a butterfly. In butterflies, as in most animals, sex is determined by the chromosomes. Females have two X chromosomes (XX) and males have just one (XO). Butterfly sperm contains either an X or no-sex chromosome.

      In this marsh fritillary butterfly (Euphydryas aurinia) the sperm that originally fertilised the egg contained an X chromosome so the offspring was due to be XX, female. But after the very first cell division into two, one of the XX cells (female) somehow lost an X and became XO (male). Throughout the many millions of further cell divisions in the growing caterpillar and during metamorphosis in the chrysalis the right-hand side of the insect stayed female while the left-hand side had become male. When the final adult butterfly emerged from its pupa, it continued to be right half female and left half male.

      Gynandromorphs are very rare and unlikely to survive. Neither male nor female sexual organs are functional. Some striking butterfly specimens occur where males and females have different wing patterns. In the case of the marsh fritillary, males are significantly smaller than females. This specimen was reared as part of a genetic study. In the wild all it could have achieved in life would have been a terminal spiral flight.