Some Biology of Insects

David Beeson, October 2022

On land, they are everywhere – the teeming hordes of life. So, their biology and lifestyles must be a big evolutionary success. In the UK we have over 22 000 species of insects in all manner of shapes and sizes. They range from the inquisitive dragonflies, through the singing grasshoppers with world record legs, to the fluttering butterflies, annoying flies destined to die in inaccessible light fittings and helpful bees. We see them scurrying behind the loo (silverfish), biting our bodies (lice and fleas), gobbling our crops (aphids) and pollinating our runner beans (bumblebees). Thunderbugs might live in the soil, but seem to enjoy dying inside framed pictures, scale insects hardly ever move. Some insects walk on water (water scorpion), nibble us as we sleep (bed bugs) and delight us with their nocturnal visits (moths).

Like the spiders and crabs, insects have a chitin skeleton on their outside. Chitin, a tough polymer, glucose-amine chemical, is slightly flexible yet inhibits any increase in size. So, insects, to grow, need to regularly shed their outer chitin layer, grow rapidly in size by inflating themselves, and then the new outer layer hardens. Just as we attach muscles to our rigid bones, insects attach theirs to their exoskeleton. But, as the new external skeleton only slowly hardens they will be unable to move to escape predation when they grow.

While growth is an issue with a chitin shell, it is a versatile material and can be formed into strong, light and flexible wings, leg joints to allow movement and mouthparts to chomp food.

Water loss is always a problem for terrestrial animals and plants. Chitin can be made waterproof, yet that will inhibit breathing (gas exchange). So, many land insects have breathing holes (spiracles) in their outer layer that are linked to air-carrying tubes (trachea) that penetrate all the internal organs. (When dissecting a locust under water, to support delicate body organs, these silver-looking air-filled tubes are easily seen.) Breathing movements of a locust’s upper and lower abdomen are easily viewed as they concertina to pump air around their bodies.

Aquatic insects will often employ gills, linked to trachea, to exchange gases with the water. The gills are thin enough to allow gases to pass through them, although this is less efficient than having open spiracles.

Oxygen, from the moving air, is used in cellular respiration – the release of energy from organic materials. (Carbon dioxide is a waste product and will flow out via the trachea.) With small body volumes any heat generated is easily lost, so insects are mostly working at the same temperature as the local environment – they are said to be cold-blooded and will be less active on cool days or out of the sunshine. This makes them vulnerable to predation.

With breathing tubes, insect blood does not usually carry oxygen, so they have no red blood cells with haemoglobin. And, with a low metabolism, insects do not need blood to remain in high-pressure blood vessels – it often slowly flows in spaces between body cells. Nor does the blood carry waste chemicals to kidneys – they do not exist as separate organs, but waste is passed out by a tubular system.

I once kept locusts for laboratory experiments and was horrified how younger animals would eat the old animals while the latter was still alive. An extreme example of recycling. We also had some reptiles and amphibians which were fed on house crickets. Some of the small insects escaped, set up home in the heating and ventilation pipes and sang to us until they ran out of food. In a similar vein, our garter snakes went walk about in the open-plan laboratories to the consternation of the chemists! But, ending on a happy note, my then president of the Biology Society (Clare Doswell) was unhappy at introducing Marwell Zoo’s snakes at a meeting. I asked another student, unknown then to her, Chris Wilmarsh to aid her – they were later married.

Some facts about insect diversity.

Silverfish are Bristletails and are considered to be primitive insects. They lack wings and rush around our homes at night seeking out food – including paper, flakes of human skin and even shampoo.

I call Thrips, Thunder-bugs, and they annoy me considerably when digging on a hot day. Yuk! They occur in soil and tend to be sap-suckers when not attacking me for a drink. Some can cause considerable economic damage to crops.

There are 500 species of Bird Lice in the UK. I clean out our bird boxes each winter to reduce their impact, as some can over-winter and will await new nesting arrivals in the spring.

While I attempt to control lice numbers, I protect Lacewings. These delicate-looking insects have long green bodies and oval wings … and their young eat aphids / green and black flies. Amazingly, lacewing larvae lack a functioning anus, and so they store up their undigested food until they moult for the last time and gain an anus – finally relieving themselves of a comparative big dollop of poo expelled with anal gases! Lacewing females are said to lay around 300 eggs, with each larva consuming up to 10 000 aphids. If you grow vegetables – look after your lacewings. We find hibernating lacewings (and ladybirds) hiding in small cavities in the autumn. Many survive but need capturing to allow them to escape in the spring.

Wild lions in the UK? Yes, antlions. This is an organism not regularly spotted in Britain until the 1990s, when a pair were observed on an RSPB reserve in Suffolk. The mature animal resembles a dragonfly; however, the grub has an over-sized set of pincer-like jaws and settles at the bottom of a sandy conical pit. It conceals itself with only its jaws visible. Unwary insects, that might tumble into this sandy trap, do not survive the event. Even a rapid exit up the side is ineffective as the young antlion throws sand to stop the escape. Want to see one? Visit Wells-next-the-Sea in Norfolk or Minsmere in Suffolk.

I come across young caddisflies during riverfly sampling. There can be dozens in a capture, and they are mostly hidden away inside a sandy or vegetarian case stuck together with a glue. Other types are caseless but live underwater in webs or hidden between river gravel. When mature the adults look like moths with wings covered in short hairs rather than scales.

Grasshoppers were the sound of summer holidays when I was a kid … 60 years ago, sadly. Their high-pitched stridulations now mostly evade me, yet small ones enjoy our wild meadows all through July and August. Their slower-moving friends, the green crickets, can be spotted on potted plants all summer and happily pose for photographs.

Homepage: www.nwhwildlife.org for 150+ ad-free knowledge.