throughout the nymphal period, were separated as to species and number in each 
instar recorder. In any one locality and year the collection made when most of 
the specimens were in the last three nymphal instars was the one used to deter- 
mine the hatching period of a species. In any locality only the primary habi- 
tats for the species were used for collecting. If more than one habitat was 
used at the same station on the same day, the data from all habitats were pooled 
to obtain the hatching period. 
HATCHING CURVE 
The hatching curve is formed by plotting the percentages of hatch, O to 100 
percent, according to the dates upon which they fall. In order to obtain data 
for statistically analyzing the effect of weather on this curve, it was necessary 
to assemble all the records of dated collections that showed the percentage of a 
species found in each nymphal instar. Only those collections were used that 
were made after hatching was complete and before few or preferably no adults had 
appeared. 
The next step was to prepare table 1 showing the average number of days in 
each instar for the four economically important species included in this study. 
These were Melanoplus sanguinipes (F.), M. bivittatus (Say),M. differentialis 
rum ( 
(Thomas), and M. femurrub De Geer). The averages were obtained from 
hundreds of individual rearing records of the four species at different constant 
and varying room temperatures. All through the development and use of this 
method of forecasting the hatching period, it is the averages that are considered 
and only approximations that are expected. Six instars are recognized for all 
four species, although this number will vary from six to seven for differentialis 
and five to six for the others. 
For an explanation of the method used to determine the hatching curve for 
any one species in a given collection, an example is discussed step by step. To 
be of use the collection must have been made on a single date and be representa- 
tive of the general infestation in the vicinity of a weather station. 
The information utilized in the example was obtained from collections of 
differentialis made in five fields in the Belle Fourche area of Butte County, 
S. Dak., on July 11, 1946. These collections were pooled and identified as to 
species and the numbers in each instar. The necessary steps are outlined in 
tables 2 to 4. 
In item 3, table 2, the number of specimens in each instar has been con- 
verted into a percentage of the total number of specimens collected. Item 1 
shows the average number of days in each nymphal instar. There were, in this 
example, no adults present in any of the five collections, but all six instars 
were present. 
Proceeding with the method, certain logical assumptions are necessarily 
made. First, a base date such as May 31 is arbitrarily selected, and the date 
of the collection, July 11, can now be considered as being 41 days after May 31. 
In table 1 the total number of days for nymphal development of differentialis 
is 42. Since sixth instar individuals were present in the collections, then 
O percent hatch (table 3) can be considered as being true on that day, 41-2 or 
