660 - Heredity and Evolution 



special mixture, called royal jelly, develops 

 into a fertile female, which eventually may 

 migrate and become the queen of a new 

 colony. But if the female larva receives an 

 ordinary mixture of nectar and pollen, she 

 develops into a sterile worker. The youngest 

 of the workers serve as nurses, preparing the 

 brood chambers and feeding the larvae; the 

 older ones are "house workers," which clean 

 and guard the hive, make wax for the honey 

 cells, and supervise the storage of food; while 

 the oldest workers are the "field bees," re- 

 sponsible for gathering pollen, nectar, and 

 water for the whole community. Probably 

 the development of the different kinds and 

 castes among socialized insects depends upon 

 a delicate balance between chromosomal, nu- 

 tritional, and hormonal factors. However, 

 more research must be carried on before this 

 balance can be defined in very precise terms. 



Insect Behavior. There is a wealth of evi- 

 dence that indicates that the behavior pat- 

 terns of insects (and other invertebrates) are 

 largely "instinctive." The behavior consists 

 mainly of unconditioned (unlearned) re- 

 sponses, performed automatically in the same 

 manner each generation — simply because the 

 animal is born that way, or gets to be that 

 way during the normal course of develop- 

 ment (p. 4(18). Thus a certain kind of solitary 

 wasp always builds a certain kind of nest 

 even though this individual has never had 

 the slightest contact with its parents or any 

 other wasp; and there are many other exam- 

 ples demonstrating such behavior among 

 various invertebrate species. 



Despite the well-established fact that in- 

 sect behavior is largely "instinctive," the ex- 

 tensive experiments and observations re- 

 ported by the Austrian zoologist Karl von 

 Frisch prove that the honeybee, at least, 

 possesses some capacity to form conditioned 

 reflexes. It thus can learn on the basis of 

 experience. Von Frisch was able to condition 

 worker bees to choose between cards of dif- 

 ferent colors and to substitute one color for 

 another while seeking the sugar solutions 

 that were used as bait. These studies also 



show that bees have a fairly elaborate system 

 by which they communicate food informa- 

 tion to other members of the colony. The 

 scout bee, loaded with pollen or nectar, indi- 

 cates the direction and distance of his find 

 by performing a series of dancing movements 

 as soon as he gets back to the hive. Dancing 

 in small quick circles indicates that the food 

 is nearby and can easily be found. But for 

 distances greater than about 75 yards, the 

 scout also gives the direction in terms of the 

 position of the sun — by straight-line dances 

 that intervene between the circles. Straight 

 upward movements mean "go toward the 

 sun"; straight downward, "away from the 

 sun"; while various angles toward and away 

 from the sun are given roughly by the angle 

 of the runs, with reference to the vertical 

 direction. Moreover, the communication sys- 

 tem works even on cloudy days, since the eye 

 of the bee is sensitive to ultraviolet light, 

 which partly penetrates the clouds. 



PHYLUM ECHINODERMATA 



The Echinodermata constitute a unique 

 group of entirely marine invertebrates. Star- 

 fishes (Fig. 32-33) are perhaps the most famil- 

 iar representatives, but there are some 5000 

 other living species. These include the sea 

 urchins (Fig. 32-33), sea lilies (Fig. 32-33), sea 

 cucumbers (Fig. 32-33), and brittle stars (Fig. 

 32-33). Also fossil remnants of many extinct 

 species have been found in formations dating 

 from the Cambrian period. Many modern 

 echinoderms are sedentary bottom dwellers; 

 and no terrestrial species have been evolved. 



Characteristics of the Phylum. Echino- 

 derms are triploblastic, nonsegmented ani- 

 mals with a well-developed coelom and a 

 mouth-to-anus digestive tract. However, the 

 anus is often rudimentary and it may even 

 be absent. Accordingly, egestion through the 

 mouth is not uncommon, at least among 

 adult echinoderms. 



The name of the phylum (Gr., echinos, 

 "hedgehog"; derma, "skin") refers to the 

 unique structure of the body wall and skin. 



