PLECOPTERA NYMPHS OF NORTH AMERICA 7 
anal gills may likewise be present. Pteronareys possesses gills on 
the underside of the thorax and on the first two abdominal seg- 
ments. Pteronarcella has gills on the first three adbominal seg- 
ments in addition to the thoracic gills. Acroneuria and the larger 
species of Perla possess copious thoracic gill tufts, and some of 
the species are also provided with one pair of anal gills. Perlesta; 
Neoperla and Perlinella have both thoracic and anal gill tufts. 
Atoperla has long gill tufts under the thorax but no anal gills. 
It is impossible to say what correlation exists between habitat 
and presence or absence of gills, or type of gills, until we have 
more detailed information about the habitat and behavior of the 
various species. 
LIFE CYCLE AND METAMORPHOSIS 
It is only within recent years that attempts have been made to 
observe the complete life cycle of stoneflies. The time required 
to complete the postembryonic development varies in different 
Nemoura vallicularia Wu 
Tire Dates of Actual widths | Computed width 
molting of head* of head 
1 July 2 Qe2o Diao 
2 July 6 2°50 20 
3 July 11 2.66 eT 
4 July 18 2.75 e219 
5 July 23 3.00 ook 
6 August 1 3.25 3.3 
7 August 8 3.50 3.6 
8 August 13 4.00 3.9 
9 August 22, 4.25 4.2 
10 September 4 4.50 4.5 
tt September 12 4.75 4.8 
12 September 24 5.00 sea 
13 October 2 5.50 5.4 
14 October 14 Delo 5.7 
15 October 23 6.00 6.0 
16 November 11 6.50 6.4 
17 November 30 Gi He 6.8 
18 December 21 7.00 Tee 
19 January 10 7.50 7.6 
20 January 30 8.00 8.0 
21 February 29 8.50 8.5 
22 March gil 9225 9.4 
Transformation| March 29 10.00 9.9 
* “Hach division on the micrometer was 160 micra and was taken asa 
unit of measurement. The ratio of increase in the width of the head be- 
tween two molts, was found to be 0.95, from which a series of computed 
widths of head in the instars was made to compare with the actual widths 
taken from the specimens.”’ 
