INSECTS. 



239 



they advance their tunnels at the rate of about half an inch a day Little 

 by little the jaws cut and tear away the wood, and at last when the ho] 

 is large enough it is stocked with eggs and pollen. 



The bumble or humble bees are social forms whose large bright yellow 

 and black bodies and droning hum are familiar to all. This hum is a true 

 voice. On a preceding page mention was made of the mechanism of 

 breathing of insects, and in the bees the act of breathing also make- the 

 note. Each of the little apertures on the side of the body through which 

 the air for respiratory purposes passes has a membrane which vibrates in 

 the current just as do the vocal cords of man, and like them it is subiect 

 to the control of the insect. 



One of the most interesting fields of natural history is that which 

 shows the relations of insects to plants. Of late years it has been culti- 

 vated most assiduously chiefly by Darwin and Hermann Miiller, and in our 

 own country by Professor William Trelease. It has been conclusively shown 

 that many flowers will not ripen seeds unless pollen be brought them from 

 some other flower. This transfer of pollen is accomplished by insects 

 which, in their search for honey, visit flower after flower and plant after 

 plant, and oft-repeated experiments have shown that if a plant be covered 

 with gauze so that no insect can visit it, no seed will be produced. These 

 relations of plants and insects have produced many wonderful modifica- 

 tions, in each the sole purpose of which is that the pollen will be deposited 

 on such a part of the insect that on visiting another flower it will be 

 brushed against the proper part for fertilizing the seed. 



The bumble-bees are the insects most effective in fertilizing the clover- 

 blossoms. Now no bumble-bees occur as natives in Australia and New 

 Zealand, and so when the colonists tried to raise clover, their endeavor 

 did not meet with success. The seeds germinated, grew, and blossomed, 

 but no seed was ripened, and so the stock died out. Again and again ti 

 attempt was made, and with the same result, until at last the experiment 

 was tried of introducing bumble-bees from Europe. This was tin- one 

 thing needful. The bees thrived and multiplied, and the clover i. 

 seeds itself just as it does with us. 



The bumble-bees make their nests beneath the surface of the ground. 

 If the queen can find some deserted hole, like the burrow of a mole, she 

 occupies it ; but if not, she excavates one for herself. In this -she deposits 

 a mass of pollen 'and honey, and in this places some egg&. As soon as one 

 brood is thus provided for, another is started in the same way. and so on 

 throughout the summer. The larva?, on hatching from the egg, feed on 

 the provisions at hand, and bore in all directions in the mass. At last the 

 time comes for the change to a pupa, The larva then spins a silken 





