510 The Outline of Science 



mouths. Young dragonflies propel themselves through the water 

 by means of the forcible expulsion of water from the end of the 

 food-canal. Insects walk, run, and jump with the quadrupeds, 

 fly with the birds, glide with the serpents, and swim with the 

 fishes. It is often asked how a fly contrives to walk up smooth, 

 perpendicular surfaces, and one answer is that a vacuum is made 

 below a little soft pad which is present on the foot. Another 

 explanation is, that there seems to be a slight exudation of 

 adhesive moisture from the foot. Beetles, which have relatively 

 strong legs, very different from the weak legs of a butterfly, can 

 run with considerable speed, while many insects one has only 

 to think of a flea or a grasshopper are pre-eminently leapers. 

 The most primitive insects, the spring-tails and bristle-tails, are 

 entirely wingless, but a spring-tail is an expert jumper. It has 

 at the end of the body an effective leaping apparatus consisting 

 of two elongated prongs, which are bent under the abdomen and 

 pressed down, affording such a leverage when the retaining 

 "catch" is released that the insect springs forward a relatively 

 long distance compared with the size of its body. 



From great leaps to the beginnings of flight is an under- 

 standable step in progress, and most insects are fliers. There 

 are many patterns of wings, but essentially they are lightly built, 

 mere flattened sacs of skin, often transparent and fragile, but 

 beating the air with an extraordinarily rapid motion. It has been 

 calculated that a fly makes 330 wing-strokes in a second, a hum- 

 ble-bee 240, a wasp 110, a dragon-fly 28, and a butterfly 9. The 

 rapidity of the movement produces a hum or buzz. Bees and 

 wasps have two pairs of membranous wings, but the fore-wing 

 and the hind-wing on each side act as a single organ, for the 

 hind-wing has a row of minute booklets which fit into the curled- 

 over posterior edge of the fore-wing and lock the two wings to- 

 gether. In dragon-flies the two wings are not attached, but the 

 two pairs are co-ordinated by the action of very strong muscles, 

 and the larger dragon-flies are excellent fliers. They are prob- 



