1940] 
Wing Pads of Pteronarcys 
113 
pads. Formation of a disc and its subsequent eversion also 
occurs in holometabolous forms. Regarding Microcentrum 
latifolium (Orthoptera) , Tower reported that the first in- 
star has well developed wing pads, which everted in the 
embryo but do not become external until the first nymphal 
moult. He does not note first occurrence of lacunae. 
Sulc (1911) sectioned the early instars of Philaneus linea- 
tus (Horn.) in order to reconstruct the tracheal system. He 
found that in the first instar tracheae do not extend to the 
site of the wing pad, which is later invaded by two tracheae 
in the second instar. He found four longitudinal tracheae in 
the wing pad of the third instar, a pattern which is elabo- 
rated in the fourth and fifth stages. Curiously enough, he 
did not describe or figure lacunae, although they have been 
illustrated in non-histological works on wings of Homoptera. 
Beck (1920) discussed the development of the wing vein 
in Phyllodromia (Blatta) germanica; this was not histologi- 
cal, but was concerned with the development of the wing’s 
tracheal system throughout the life of the roach. Neither 
the advent nor the differentiation of lacunae is mentioned. 
In order to investigate the differentiation and growth of 
wing epithelia and to ascertain the relationship between the 
development of lacuna and trachea, Pteronarcys proteus 
Newport was chosen. This insect is a primitive plecopteran 
with at least fourteen nymphal stages. The writer was for- 
tunate in obtaining several early instars which were studied 
exclusively by serial sections, since whole mounts of entire 
individuals or of wing pads do not show lacunae. Specimens 
were fixed in Kahle’s fluid (two hours), sectioned in paraffin 
at 6p. without difficulty or loss of sections and stained with 
Mallory’s triple connective tissue stain. 
The length of the specimens is not given as it is an unre- 
liable index of a particular instar. Body length varies among 
fixed specimens : some are caused to telescope by the action 
of the fixative, others are relaxed at full length. Head 
width, however, enables one more definitely to determine the 
stadium to which a specimen belongs. 
First Instar 
Head width, .408 mm. A single specimen was obtained 
and sectioned. The wing anlage is not a disc of cells de- 
