302 ECOLOGY 



Slope, kind of soil, and soil moisture are factors governing the 

 deficiency or absence of eggs. A deficiency or excess in any one of 

 these respects decreases the number of eggs laid, or causes them not to 

 be laid at all. The animals are in the condition for egg-laying for but a 

 short period. 



d) Method of selection. — It has been determined by opening holes 

 that eggs are not laid in all, and in one case the first holes made by the 

 female were empty. This would tend to show that the female beetle 

 tries the soil before laying the eggs, but I have not been able in other 

 cases to determine whether the first holes contained eggs or not. To 

 determine this, it would be necessary to watch a female all of the time 

 during several days. 



3. LAW OF TOLERATION (55) 



Repeated experiments with several species have shown results 

 similar to those shown in Table LXVIII, and we have concluded that the 

 egg-laying place of the tiger-beetles is their true habitat. The tiger- 

 beetles which lay eggs in soil do so only when the surrounding tempera- 

 ture and light are both suitable, the soil moist and probably also warm. 

 The soil must satisfy the ovipositor (egg-laying organ) tests with respect 

 to several factors. Egg-laying, the positive reaction, is then probably 

 a response to several factors. Furthermore, after the eggs are laid, the 

 conditions favorable for egg-laying must continue for about two weeks 

 if the eggs are to hatch and the larvae reach the surface. The success 

 of reproduction depends upon the qualitative and quantitative com- 

 pleteness of the complex of conditions. This complete complex is called 

 the ecological optimum. The negative reaction, on the other hand, appears 

 to be different. The absence of eggs, the number of failures to lay, and 

 therefore the number of eggs laid in any situation, can be controlled by 

 qualitative or quantitative conditions with respect to any one of several 

 factors. The presence, absence, or number of eggs laid may be governed 

 by a single factor. 



For example, all other conditions being optimum, moisture may 

 control the presence, absence, or number of eggs laid. If the moisture 

 be optimum, the maximum number of eggs will be laid. If it is too 

 great few or no eggs will be laid. This factor then controls according 

 as it is near the optimum, or near either the maximum or minimum 

 tolerated by the species. It is, however, not necessary that but a single 

 factor should deviate; the effect is similar or more pronounced if several 

 vary. 



