September 2015

Cockchafer

Feelers, or more correctly antennae, are yet another thing that set the insects apart from other creatures (though crabs and lobsters make good use of them too).  They are antennae rather than feelers because they do more than provide the insect with a sense of touch.  In most insects they are primarily used to provide a sense of smell, though insects with poor eyesight that forage at night like cockroaches may also use them to avoid running into things.  They can also detect vibration and sound, as well as electrical charges, and heat, to name but a few.  Most insect antennae are of one basic pattern, that is, they are composed of a number of segments.  The first segment is attached to the head and is a moveable joint controlled by muscles so that the insect has control of which direction they point.  The second segment contains something called a Johnston’s organ.  This essentially contains movement sensors that allow it to detect motion or vibrations of various frequencies.  Some are tuned to the frequency of a mate’s wing beats, or to the stridulation of a potential mate.  (Stridulation is a fine word, but it just means the sort of noise made by a grasshopper or cricket.)  In hawkmoths, movement detected in the second segment corresponds to angular changes in flight which helps the moth make complex manoeuvres.  The waggle dance performed by honey-bees to communicate to other workers the location of good forage creates a varying electric field that alternately repels and attracts the antennae.  This allows workers out of immediate sight or ‘earshot’ to still read the waggle dance.  Depending on the type of insect each antenna can have one or many more segments beyond the second segment and in the main these contain sensors that detect smell.  These are called sensilla and are tiny hair-like protrusions on the cuticle that collect the smell molecules.

Short-winged Conehead

Antennae come in a bewildering array of shapes or sizes that have developed according to how the insect uses its antenna.  Many male moths have large feathery antennae and their prime purpose is to detect pheromones emitted by the females.  They do that very well and the feathered nature of the antennae increases the surface area available for sensilla so that they only need one or two molecules of the pheromone to indicate that a receptive female is nearby.  It has been estimated that a male sat on a goal post at one end of a football field can detect the presence of a female sat on the goal at the other end (though nobody said whether there was a match on at the time or not).  Other insects such as the bush-crickets use the length of their antennae as a means of increasing the surface area, and in some the antennae may be 4 or 5 times the length of the insect itself.  Hoverflies have only 3 segments to their antennae, but the third segment is shaped a bit like a table-tennis bat to receive sound better.  It also has a hair-like projection on the third segment called an arista that presumably is for detecting different tones.

Muslin Moth

Antennae can be useful to entomologists as well.  Firstly they can be used to indicate insect groups.  For example many hoverflies mimic bees or wasps, but a quick look at their 3 segment table-tennis bats easily separates them from the 12 segment antennaed wasps and bees.  Another use can be to separate male and females.  The cockchafer male pictured above has 7 ‘leaves’ on the end of his antennae, whereas the female has only 6.  All the better to detect her pheromones with no doubt.  Male wasps and bees have 13 segments in their antennae, females only 12 – though the chance of getting a wasp to keep still while you examine it with a hand lens and count the segments is small indeed, but if it jumps up and stings you you’ll know it’s a female as males can’t sting.

The insect world is dominated by chemical signals for various reasons such as defence mechanisms or to lay trails as ants do.  In order to understand what the signals are and how the animals use them research entomologists use a technique called electroantennography that allows them to pick up the electrical signals generated by the antennae when exposed to certain chemicals. Nature is wonderful, but the people who find this stuff out are pretty special too.