ARTHROPODA 309 



cob-web spider, make a complex and irregular mesh-work of fibres running in all 

 directions; others spin a web similar to the last with the exception that at one point 

 it is continued into a tube into which the spider retreats for hiding. The webs of 

 these spiders are for the 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 number 

 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 fasten- 

 ing 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, harvest-men or "daddy- 

 long-legs," and many parasitic or otherwise degenerate forms. 



336. 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 pollinating flowers 

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

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

 flowers by insects deemed a common and important phenome- 

 non by botanists ? 



10. Can you find any recorded instances of what may be 



