76 MISC. PUBLICATION 336, U. S. DEPT. OF AGRICULTURE 
Key TO LARVAE (FOURTH-STAGE) 
The genera | are separated in couplets 1 to 9, and the species in the 
couplets that follow. Air-tube lengths, when mentioned in connec- 
tion with the species, are given in multiples of the diameter of the 
base of the tube. Mounted larval skins become flattened by pressure 
of the cover glass, and allowance must be made for this in estimating 
the proportions of the tube. Variations may occur in the number of 
branches as given for certain hairs; that is, a normally double hair 
may occasionally be single or triple, or a single hair double. In ex- 
Ygver head fair------------ - 
| 
Lower head fair ~---~-— 3 pea Avrenna/ pair 
ay yi 
Vy ia F l 
\ : ly} ay SSS S5 Antenna 
} iy ] Yj 
Voy Wy 
\\\ ty Vy HEAD 
ETE THORAX 
Lateral! hair----= 
Subaorsal palr~---~ Ls 
ALLOIIEN 
Lateral COM ne 
Of G74 se 2GIPIELIT Z 
Anal segrieri7 —-— << 
herntra/ brush — \ 3 VEN pecme 
_Bz>— A Wy 4 
Ana glle-—— ne \ NIK 
f \ eX 
\--~Sentra/ tutes 
/ A 
Dorsal 7uf?--—-- AEPEPOUONEN, SOC S ie PPE Pecten 
FIGuRE 20.—Larva of Culex quinquéfasciatus with the parts named (Howard, 
Dyar, and Knab). 
amining flattened specimens care must be taken to distinguish between 
the upper and lower surfaces of the body. The terminology of the 
larval parts is shown in figures 20, 21, and 22. 
Some difficulty is often ¢ experienced in distinguishing between third 
and fourth instars, and no certain rules can be given except for dif- 
ferences in size and in the width of the head capsule. In Anopheles 
larvae a comparatively wide collar of dark sclerotin (chitin) at the 
base of the head is usually distinguishable in the third instar. In 
larvae of Psorophora and Culex, and in those species of Aedes in which 
the anal segment is completely ringed by a plate of sclerotin in the 
