8 THOMAS SAY FOUNDATION 
species. In some of the smaller forms, such as the Nemouridae 
and Capniidae, the life cycle is completed in one year while in 
the larger forms, such as Pteronarcys, Acroneuria, and certain 
species of Perla, two to three years are required for each genera- 
tion. Samal (48) estimates that Perla abdominalis requires three 
and one-half to four years to complete its development. 
The number of nymphal instars may vary considerably but in 
the forms studied it is twenty-two or more. Tillyard (54) says, 
‘‘The number of instars is not known for certain, but appears 
to be usually six.’’ 
Wu (57), in studying the life history of Nemoura vallicularia, 
recorded twenty-two instars ‘‘from the hatching of the naiads on 
July 2, 1921, to the emergence of the adults on March 29, 1922.’’ 
The table on page 7 is taken from Wu. 
Samal (43) found that Perla abdominalis also passes through 
twenty-two nymphal instars, which, according to his estimates, ex-. 
tended over a period of three and one-half to four years. His data 
are given in tabular form as follows: 
Samal’s Table of Larval Stages of Perla abdominalis 
Number of Number of Length of Length of 
Stage segments segments body without body with 
in cerci in antennae cerci cerci 
1 3 9 — 0.8—- 1mm 
2 3 9 — 1 —1.5 
3 4 11 — 2-83 
+ 5 12 — 2.5- 2 
5 6 14 _- 2.5- 2 
6 7 1 — 2.7— 3.2 
7 9 16 (?) — 3 -— 3.8 
8 10 sae — 3.6- 4 
9 11 19-20 — 3.8- 4.2 
10 13 23-24 — 4 — 4.8 
11 17 28 — 4.5- 5 
12 19 31 — 5.3- 6 
13 20 33-37 -— 6 -7 
14 26-28 36-39 4.6- 5mm 8.2- 9 
15 32 33-41 5.5- 6.7 9.5-10 
16 35 44-47 7 -8 12 -13 
17 35-40 48-52 8 -— 9.4 12 -14 
18 36-40 58 9 -10 15 -17 
19 44-45 60-65 10 —-ll1 15.14 
20 240-43 60-67(70)| 914 —15 22 -23 
v10 -11 17-19 
21 49-50 79-85 918 -22 25 —28 
G12 -15 18 -21 
104 48-50 79-91 925 -28 35 40 
S19 —24 19 —24 
23 50-54 85-96 930 -32 44 -48 
