120 
specimens of cryptostigmatid mites occurred in 
the top 2.5 cm of the litter and soil. Unlike the 
north temperate regions, in the arctic soil ani- 
mals cannot descend to escape the winter freeze. 
By remaining near the surface, they experience 
an earlier onset of activity in spring and warmer 
temperatures during the summer season. 
3. Phenology. One of the striking features 
of tundra ecology is the synchronous emergence 
of adult insects (MacLean and Pitelka 1971; 
MacLean, in press). The peak of emergence at 
Barrow, particularly of the conspicuous Diptera, 
usually falls in the second week of July approxi- 
mately one month after melt-off. We were inter- 
ested in comparing emergence patterns at Prud- 
hoe Bay where snow melt-off is generally earlier 
and summer temperatures warmer. 
Cumulative 
Captures 50 
(% of Total) 
Date 
Fig. 1. Derivation of the figure used to describe 
seasonal pattern of insect activity. Central line 
represents date of median capture; bar encloses 
central 80% of captures; total length of horizon- 
tal line indicates total period of capture. An 
open-ended line (see Fig. 3) indicates that the 
first (or last) capture occurred in the first (last) 
period sampled; a closed line, as shown here, 
indicates that at least one sample period with no 
captures preceded (or followed) the first (last) 
capture. 
The 2 years of observation at Prudhoe Bay 
yielded very different results. In 1971 melt-off 
was nearly complete when observations began in 
early June. The average of the daily mean air 
temperatures recorded at the BP-Mukluk based 
camp during the first week of June was 6.1°C. 
In 1972 melt-off was delayed by at least 10 days 
and occurred at approximately the same time as 
melt-off at Barrow. The average daily mean 
temperature for the first week of June 1972 was 
0.5°C. Thus, invertebrate activity began much 
earlier in 1971. Air temperatures in the two 
seasons were then roughly comparable until late 
July, when 1971 was warmer than 1972. August 
1972 was much warmer than August 1971, 
reversing the earlier trend; thus, temperature 
differences averaged out for the season. 
All taxa examined showed a significant delay 
in emergence in 1972 relative to 1971 (Figs. 
2-7). This delay approximately equals the differ- 
ence in melt-off in the 2 years. This again 
demonstrates the unimportance of photo-related 
cues as a timing mechanism for arctic tundra 
invertebrates. Rather, it appears that emergence 
follows the completion of a certain amount of 
metabolic activity which begins at or soon after 
melt-off. 
No consistent between-habitat differences 
are evident in the timing of emergence. In most 
cases, the dates of median capture of any taxon 
on all plots fell within a 5-day period. The 
synchrony of emergence is particularly evident 
in the Tipulidae, where 80% of the captures 
occurred within 10-day periods in both years 
(Fig. 2). Craneflies are important prey for breed- 
ing sandpipers, and the timing of sandpiper 
breeding activities has probably evolved so that 
the hatching of sandpiper young coincides with 
the appearance of their major prey. The 
synchronous nature of the emergence allows 
little room for error. The period of cranefly 
Diptera: Tipulidae 
Plot N 
4 97 rot} 74 
972 i$ 236 
97) f=} 64 
5 
972 a 216 
os 1971 = =— 162 
972 + — { 626 
l =) een | ee Is ea ll i] 
Fig. 2. Seasonal distribution of “sticky-board” 
captures of adult craneflies (Diptera: Tipulidae) 
in 1971 and 1972. 
