306 ZOOLOGY. 



purpose of catching flies and other insects on which the animal feeds. The 

 trap-door spiders make a tunnel in the ground which they line with their 

 secretion ; a door is woven which is so covered with materials like those 

 about the nest that its presence is effectually hidden. A considerable num- 

 ber of spiders do not spin proper webs, but use their secretion merely in 

 forming cocoons for their eggs, or in binding together objects to make 

 a home. This wonderful secretion is used by the spider in many other 

 ways than in the capture of prey and the making of a nest. By means of 

 it some of the spiders make a near approach to flying. A spider may 

 bridge the space from one object to another either by fastening one end 

 of the strand and hanging at the other, or by sitting still, he may allow 

 the free end to float out until it becomes attached. In some cases at least 

 it is known that, by spinning thus loose silk in abundance, the weight of 

 the spider may be readily carried by the action of the wind upon his 

 silken sails. 



The chief economic importance of spiders lies in their habit of preying 

 on various insects, of which they destroy considerable numbers. 



The Arachnida embraces a number of other orders including less 

 important or less easily observed animals, as the mites, certain ticks, har- 

 vest-men or " daddy-long-legs," and many parasitic or otherwise degen- 

 erate forms. 



328. Suggestive Studies, for Field and Library. 



1. Dimorphism and polymorphism in insects. 



2. Protective adaptations in insects. 



3. What senses seem most used among the insects? 



4. Report on observed signs of intelligence among arthro- 

 pods. 



5. Is there any evidence of power of communication among 

 the social insects, as the ants? 



6. Courtship among the spiders. 



7. Spiders' webs : form, position, efficiency, mode of con- 

 struction. 



8. There are some insects which have wings during a por- 

 tion of their life but lose them later. Investigate the condi- 

 tions and find an explanation. 



9. Report an observed instance of insects fertilizing flowers 

 (i. e., transferring pollen from one to another). How is it 

 effected? Why does the insect do it? Is the fertilization of 

 flowers by insects deemed a common and important phenome- 

 non by botanists? 



