42 2 GENERAL PRINCIPLES 



covered up with the mud or washed away. The seed remains 

 on the tree until it has germinated and developed a heavy 

 hypocotyl about a foot in length. It then falls and strikes deep 

 enough into the mud so that it remains upright as if planted. 



855. Among the social Hymenoptera the young are cared for 

 as carefully as they are by the higher vertebrates. In other 

 cases, however, the young are never seen by the mother, and, 

 indeed, in many cases, the mother dies before they are hatched. 

 But even in such cases the mother may make elabora;te provision 

 for the young. The Pronuba will illustrate this point, but 

 another more remarkable examule is frequently quoted. Many 

 of the solitary wasps (Sphegidae) excavate channels under ground 

 or build mud nests under the eaves of houses. These nests are 

 then filled with spiders or other insects which the wasp stings in 

 such a fashion that they are paralyzed, but not killed. An egg 

 is then deposited by the wasp and the nest sealed up. The 

 larva soon hatches and feeds upon the spiders. If the spiders 

 had been killed decomposition would soon set in and the result 

 would probably be the death of the wasp larva. If the spiders 

 were not sufficiently stupefied they would probably kill the larva. 

 It is, therefore, of great importance that the spiders should be 

 stung in a very particular manner. But the wasp never returns 

 to the nest and cannot know how it fares with her offspring. 

 If, however, her work was not well done she will have no off- 

 spring to inherit her careless ways. Our wonder and admira- 

 tion of the instinct and skill of the successful wasp are increased 

 when we consider that the proper stinging of the spiders is not a 

 deed that is performed with calm deliberation, for the spiders 

 are also armed and are bold and skillful fighters. The wasp is 

 compelled to place carefully the paralyzing thrust in the midst of 

 a desperate conflict. 



856. Sexual Dimorphism.^ — Reference has been made to 

 sexual dimorphism (page 353). This is more general among 

 the higher animals. When there is a notable difference between 



