A warm summer’s evening. A walk through a flower-filled meadow. The rhythmic chirping of grasshoppers and the constant humming of crickets.
The noises that these insects make are an essential element of our summer and a vital part of the insects’ ecology.
The orthoptera – literally ‘straight winged insects’, the grasshoppers and crickets – are unmistakable. In the UK, they are a group of fifty-or-so, relatively large insects with elongated hind legs for jumping and a famous ability to make sounds.
The skilled naturalist can identify species from a distance – each species’ song is distinct and recognisable. Crickets produce sounds by rubbing their forewings together and grasshoppers rub their hind legs across their forewings. Each of these body parts is modified to create and project the sound. Grasshoppers have a row of pegs on their legs and a raised vein on their wing, while crickets have various structures on their wings to vibrate and amplify their song.
This difference in the way that sounds are produced leads to the distinctiveness that makes species recognisable. Dark bush-crickets make a short, simple, repeated chirrup, wood crickets have a quiet, purring song and meadow grasshoppers create a burst of varied sounds like a ‘dry chuckle’.
At first thought, it seems obvious that the purpose of this array of sounds is to attract a mate. Like birds, males are much more vocal than females, and it is only adults that are able to sing.
But, as with most things ecological, things are much more complicated.
A singing male is more likely to get eaten or discovered by a parasite than a quiet one. Their songs have therefore evolved to make locating them difficult, and their cryptic colouration and evasive behaviour gives them added protection. But, if they are difficult to find by predators, then prospective mates will also have trouble. So female crickets are very good at using their widely-spaced ‘ears’ (on their front legs) to sense the direction of the sounds, and they can feel vibrations through the substrate that they are standing on to further aid them.
Different species have different songs so that females are able to recognise their own kind and a risky journey to mate isn’t wasted. This is especially important in locations where there are multiple species all singing away so, in such cases, songs tend to be more variable between species.
A male cricket or grasshopper’s greatest rival is another male of the same species. They are both competing for the same source of food and for the same female mates. Their song acts as an important advert to females – the more attractive their song, the more likely they are to mate and pass on their genes to the next generation. Females of different species have demonstrated preferences to males with louder, faster, higher or lower songs. This can be an indication of the male’s size, age and general health, as well as their ability to produce an attractive song.
And, of course, there are exceptions. Male oak bush-crickets, the bright green cricket with long antennae that often find their way into our houses in mid-summer, do not rub their wings or legs together to create their sound. Instead they drum one of their hind legs on the leaf on which they stand, producing a short burst of sound. The Fred Astaire of the cricket world?
As we walk through the long grass of the meadow, grasshoppers leaping out of our way, we are disturbing a complex web of interactions where sounds play an important role. Stop, close your eyes, listen and enjoy.