1964] 
McCrone and Levi — Latrodectus 
23 
Latrodectus mactans is sympatric with both L. bishopi and L. vari- 
olas in Florida. It has been found in the same sand-pine scrubs as L. 
bishopi and in Torreya State Park with L. variolas. In both places 
it makes its webs near the ground. It seems to prefer ground depres- 
sions around the bases of palmetto bushes in the sand-pine scrub, and 
debris and rocks in Torreya State Park. It readily invades disturbed 
areas. 
The most striking difference between L. mactans and L. variolas 
is the difference in their rate of post-embryonic development. Both 
species spend almost exactly the same amount of time in the egg sac 
but after emergence there is a marked difference in the length of 
time and the number of molts to maturity. 
Twelve L. mactans egg sacs and io L. variolus egg sacs were 
formed in the laboratory in St. Petersburg during the summer of 
1963. The average length of time spent in the egg sac by L. mactans 
spiderlings was 29.3 zb 1.3 days (range 27-31) and the average length 
of time for L. variolus spiderlings was 29.3 zb 1.3 days (range 28-31 ) . 
Fifty of the spiderlings that emerged from an egg sac of L. mactans 
on 16 June 1963, and fifty that emerged from an egg sac of L. variolus 
on 19 June 1963, were taken for observation and rearing. Both egg 
sacs had been produced in the laboratory by individuals collected in 
northwest Florida. Each of the 100 spiderlings was placed in an 
individual, numbered container and these containers were then placed 
in an air-conditioned room where the temperature was kept reasonably 
constant around 24°C. The spiderlings received a plentiful supply 
of living fruit flies, Drosophila mclanogaster. When the immature 
spiders were large enough, they were fed as many housefly maggots as 
they would take, until they reached maturity. Originally we had 
planned to feed them adult flies, but both species showed a decided 
preference for the maggots. At no time were the developing spiders 
given water; all fluid came from the food. It was our experience that 
the addition of water is unnecessary and may encourage mold, which 
inhibits growth or may be lethal. A record was kept for each devel- 
oping spider of the number of molts and the duration of the stadia 
Explanation of Plate 3 
Fig. 18. Egg sacs (from left to right), Latrodectus variolus, L. bishopi 
and L. mactans, all Florida. 
Fig. 19. Egg sac of L. variolus. 
Fig. 20. Successive egg sacs of an individual female of Latrodectus mactans 
tredecimguttatus from Israel (in Florida laboratory culture; order unknown). 
Figs. 21-22. Latrodectus bishopi Kaston, females. (Photograph by H. K. 
Wallace) 
