20 



FIFTH REPORT OF THE ENTOMOLOGICAL COMMISSION. 



cycle two, three, or more times in a year; such insects are said to be 

 double or treble-brooded. Lophyrus abietis and other species are double- 

 brooded, while many butterflies are double or treble brooded, and the 

 Aphides have from nine to fourteen generations in a season, i. e., from 

 spring to autumn. In all cases of seasonal dimorphism or of partheno- 

 genesis there are several generations. 



Judeich and Nitsche graphically represent as follows the generations 

 of the European Lophyrus pini, with its double generations, which will 

 also apply to our L. abietis : The egg is denoted by a point ( • ), the 

 larva by a dash ( — ), the larva lying in a semi-pajia condition in the 

 cocoon, thus ( O ) ; the papa by the following mark ( m ), and the imago 

 by a cross ( + ) ; tiie time during which tUe larva is eating, by a heavy 

 dash (■■); lastly, the period of injury by the larva is placed under, 

 the time of imaginal injury above, the mark for the stage under consid- 

 eration . 



In the United States a butterfly or moth which is single-biooded in the 

 New England or northern Central States may be three-brooded in the 

 Southern or (irulf States. A generation or brood which appears and 

 ends in the summer is shorter than that which hibernates. 



Thus the summer generation of the species of pine saw-flies {Lophyrus) 

 is about four months, the winter generation about eight months. 

 Hence the length of the generation depends on the temperature and 

 climate, as does also the number of broods or generations. "This influ- 

 ence of climate is, as is well known, so considerable that a species of 

 insect which has a double generation in a certain locality, in another 

 place with a colder climate is only single-brooded, while in a warmer 

 climate it is three-brooded. An example is Hylesinus piniperda. Thus 

 also a species of insect whose generations in a certain middle location 

 is, for example, four-yearly, in a more southern situation is three-yearly. 

 A proof of this is afforded by the May beetle, which north of the ' main 

 line' is four, but south of it needs only three years to complete its 

 development. A certain species of insect may moreover in the same 

 locality in a warmer and more favorable year be double-brooded, while 

 in the next harsher unfavorable year it is single-brooded. But if the 

 checking influence of the harsh weather is less, then even in an un- 

 favorable year a second generation may begin to develop, but does not 

 complete its cycle by the end of twelve months. Hence there are in 

 twenty-four months three generations, and then arises what Ratzeburg 

 calls a ' one-and-a-half generation.' Of this Tomicus bidentatus not rarely 

 affords an example. 



