296 



METAZOAN PHYLA 



Some Lepidoptera are useful because their visits to flowers result in 

 cross-pollination. Chalcid flies and ichneumon flies, which belong to 

 the Hymenoptera, are beneficial because they attack the developmental 

 stages or the adults of many injurious insects. The egg is laid on or in 

 the host the soft tissues of which the developing larva destroys. The 

 larva of one ichneumon fly attacks the borers in shade trees, and the fly 

 is often seen laying its eggs in the trunks and larger limbs of such trees 

 (Fig. 201). It is 1}^ inches long and has an ovipositor 6 inches long, 

 with which it drills a hole through the wood into the burrow of the boring 

 larva where it deposits an egg. Many seeing this insect at work hold 



it responsible for the injury to the tree 

 which is caused by the concealed borer. 

 322. Social Insects. — Social 

 instincts are exhibited by a consider- 

 able number of insects, especially by 

 termites and some beetles. Some 

 lepidopterous larvae make communal 

 webs. In the Hymenoptera, however, 

 and especially among the ants, bees, 

 and wasps, the most striking instances 

 of insect societies occur. 



Among social insects the honeybee 

 (Fig. 202) stands out not only as a 

 type but also as that invertebrate 

 which has been most intimately 

 associated with mankind and has 

 reached the highest degree of domes- 

 tication. For ages before man learned 

 how to manufacture sugar he depended 

 for his sv^^eets very largely upon the 

 bees, and honey was an important 

 element in human diet. 

 A swarm, or society, of honeybees includes three distinct types — 

 workers, drones, and queen. In a swarm of 60,000 individuals there are 

 perhaps 200 drones and but one queen, all the rest being workers. Of 

 the three types the queen is the largest, is distinguished by the greater 

 length of her abdomen, and fives for many years. She lays all of the 

 eggs from which are produced the rest of the members of the swarm. 

 The workers are infertile females which do not normally lay eggs but 

 perform aU the labor of the society. They five for only a few weeks. 

 The drones are the males and are produced from unfertilized eggs. They 

 perform no service in the hive but exist only for the purpose of fertilizing 

 the eggs. They are relatively broader than either queens or workers, 

 intermediate in size between them, and may be recognized by their very 



Fig. 201. — An ichneumon fly, 

 Megarhyssa lunator (Fabricius), de- 

 positing its eggs in the tunnels of 

 Tremex columba (Linnaeus), on the 

 larva of which its larva feeds. Natural 

 size. (From Graham, "Principles of 

 Forest Entomology,'' by the courtesy of 

 McGraw-Hill Book Com,pany, Inc.) 

 The figure shows the characteristic 

 attitude of the insect when drilling. 



