Comparison of the North American widow species
This post is to serve as a reference for information about the North American widow spiders (mainly the black widow spiders). Much of the information and facts were gleaned from outside sources, only some is mine. The list is not complete as other info can be found elsewhere in this thread. Presentation is set up to follow the life of the spider.
Average egg sizes:
The eggs are spherical, or nearly so.
Those of L. mactans average about 0.95 mm in diameter
Those of L. hesperus average about 1.1 mm in diameter
Those of L. variolus average about 1.2 mm in diameter
Photos: normal eggs / red eggs
Time of emergence from eggsac:
Variolus takes the shortest time to emerge
Mactans is in between
Hesperus takes the longest time to emerge
Photos of slings emerging from eggsac
Average time, in days, from oviposition to emergence:
Variolus: 26.2 ±2.2
Mactans: 29.1 ±3.0
Hesperus: 30.3 ± 2.8
(Slings molt usually once, sometimes twice, before emerging from the eggsac)
Size of slings:
Mactans: smallest slings
Hesperus: small slings
Variolus: largest slings
(Bishopi also have very large slings. For example, a 1st instar bishopi is twice the size of 1st instar mactans or hesperus, and a 2nd instar bishopi is about equal to a 4th instar mactans/hesperus. Bishopi naturally produce fewer but significantly larger eggs/slings compared to the other species.)
Sling coloration:
Hesperus: many are very light (white) in color. variation is minimal when compared to mactans, and variolus to a lesser extent.
Mactans: have red, yellow, white, or tan slings. doesn't matter where they are from. some females produce red one sac and white another.
Variolus: while coloration might be similar to the other species at times, variolus look nothing like mactans as far as size and pattern go.
Bishopi: color variation is minimal.
Geometricus: color variation is minimal.
Hesperus: photo
Mactans: photo
Variolus: photo
Bishopi: photo
Geometricus: photo
Average number of days to maturity, roughly:
Hesperus: males- 100, females- 200
Mactans: males- 70, females- 140
Variolus: males- 130, females- 150
(Females mature at the 7th and 8th instars)
(Browns typically mature much faster than our native species)
Penultimate recognition:
In specimens one molt short of maturity, the area where the epigynum will later show appears much arched. Thus it is possible to recognize one in the penultimate instar. In the antepenultimate instar, the area is usually somewhat lighter than those surrounding areas, but definite recognition of females in this stage is not easy.
Distinguishing adults from subadults:
An easy way to distinguish adults from subadults is by the curvature of femurs I and IV in adults. Curvature in other leg segments such as the tibia and metatarsus is a defect at any stage in Latrodectus (although sometimes there is a normal slight curve in metatarsi I).
Males:
-Males can be sexed at the antepenultimate stage (by enlarged palps, without using microscope).
-Males can be distinguished from females as early as 3rd instar by observing palp width!
-Males typically mature at different instars in Latrodectus, smallest being the 5th.
-Adult males of the N.A. species generally show no aggression towards slings.
-Of the 3 N.A. "black" widows, mactans is the smallest and variolus is the largest (also, geometricus males are very small - photo)
Coloration and description of males:
Hesperus male:
The male has, on the carapace, a dark gray to black band along the lateral margins, and a dark band along the middle. The sternum is dark along the lateral borders, but lighter in the middle. The legs show dark annuli. The hourglass mark is not much constricted in the middle and is usually yellow rather than red, some specimens showing a slight suffusing of orange pigment. The abdominal dorsum shows a characteristic pattern of olive greenish gray alternating with light tan bands. One of the light areas appears as a middorsal band usually with orange pigment as a thin line down its center. The other light areas appear as three bands on each side, which extend along the sides diagonally down and toward the rear. The most anterior of these is hooked and procurved nears its ventral end. The male of L. mactans also shows three bands along the side but only rarely is the first one hooked and procurved. In L. variolus there are four bands. Moreover, in both L. mactans and L. variolus, the areas between these light bands are much darker than in L. hesperus.
Mactans male:
Generally, the male is similar to a fifth instar female. The cephalothorax may be all black, or the carapace may show a somewhat lighter band on either side of a median black band. The legs may be all black, or have lighter annuli retained from earlier instars. There is a median row of red spots on the dorsum of the abdomen. On the venter the hourglass mark is distinct. Encircling the anterior end of the abdomen is a white band, and on either side farther back are two additional white bands which extend diagonally down and to the rear. Seen from the side this gives the appearance of three white bands. In L. Hesperus there are also three white bands as seen from the side, but the anterior one is always hooked and procurved near its ventral end, while this is uncommon in L. mactans. Also, in L. Hesperus the background color is much lighter. In L. variolus the ground color is as dark as in L. mactans. There are four bands visible along the side.
Variolus male:
The male has a black cephalothorax and black legs, as in the female, but the legs are more likely to show the brown annuli. The dorsum shows a pattern similar to that of the female, but with the white stripes generally broader. From the side, four such stripes are visible, making this sex relatively easy to distinguish from the males of L. Hesperus and L. mactans, which have only three light stripes. The hourglass mark is divided.
Photos: normal / showing detail
Geometrics male - and remember, geometricus males are TINY!
Number of embolus coils:
Bishopi and variolus: 2
Mactans: 3
Geometricus: 4
Hesperus: varies
Character of embolus in males:
Levi found, that to some extent at least, the character of the embolus could be used to separate species. In L. variolas it is wider than, and about three-fourths as long as, in L. mactans. It makes two coils in L. variolus, but three in L. mactans. Moreover, the coils are tighter and less open in L. variolus than in L. mactans. In the type male of L. hesperus, the embolus shows two coils (as in the "curacaviensis group") though it appears longer than in L. variolus, as the coils are more open. However, the coils are tighter and less open than in L. mactans. But there is much variation among the many specimens of L. hesperus that have been studied.
Sizes:
-Of the three species, L. mactans averages smallest for both sexes. Thirty-seven males ranged from 2.9 to 5.1 mm in length, with most between 3.2 and 4 mm; 52 females ranged from 5 to 13.5 mm, with most between 8 and 10 mm.
-Latrodectus variolus has the largest males, mostly between 5.5 to 6.5 mm, with a range for 34 specimens of 4.5 to 8.3 mm. Females of L. variolus are mostly 9 to 11 mm in length with a range for 32 specimens of 7.4 to 13 mm.
-Latrodectus hesperus has large females, 59 specimens ranging from 8 to 15.5 mm, with most from 10.5 to 13 mm. Sixty-three males ranged from 3 to 6.5 mm, with most between 3.8 and 4.5 mm.
(The female brown widow is the most variable Latrodectus species in both size and coloration.)
(The large southeastern L. variolus are the largest widows in NA. There is much variation in adult size in all NA widows throughout their ranges and within local populations though. Each population consists of large, average, and small adults. Large, average, and small refers to CL and LS. The size variation is a result of specimens maturing at different instars, and instar sizes. Spiders at the same instar do not always have the same measurements.)
Picture showing how much larger the SE variolus are, compared with hesperus
Closeness of species:
Hesperus and mactans: so closely related are hesperus and mactans that they were for a long time considered to be the same species.
Variolus and bishopi: L. bishopi and L. variolus are certainly distinct species, but the close similarity between the two in morphology and their allopatric distribution suggests that L. bishopi might have been derived from L. variolus.
Sympatric species:
The only sympatric species to be observed so far are bishopi/geometricus, mactans/geometricus, and mactans/variolus. Mactans has not been seen in bishopi habitat yet. And geometricus may very well displace species such as mactans and hesperus in NA, but it is not known if there are any official reports of it. Mactans and geometricus retreats have been observed within one foot of each other, and mactans/variolus and bishopi/geometricus webs within several feet of each other.
Reaction to threats:
Most are similar in regards to fleeing to their retreat to hide, but some will present the sticky globs of glue. Geometricus will even just fall to the ground, something I rarely see other widows do.
Leg length proportion:
-Although there is much variation, for females the first leg is proportionately longest in L. variolus, shortest in L. mactans, and intermediate in L. hesperus. In males, there is very little difference between L. hesperus and L. variolus, both of which have the first leg averaging slightly longer than in L. mactans.
-Length of legs from longest to shortest in Latrodectus is: 1-4-2-3.
Some mating stuff (much can be written on the processes and whatnot):
-“Intrusive opportunistic mating” can occur between mature males and subadult females.
-Apparently there is a brief period of time after satiation prior to molting when a subadult female will allow a male to mate. Prior to satiation, an adult male is generally perceived as nothing more than prey to an immature female spider.
-Often sneaky males that spend little time courting and go straight to trying to mate are killed and eaten or killed and thrown out.
-Males often destroy the females web prior to mating and place their own silk throughout the web. This possibly helps prevent conspecific males from entering the web or maybe to warn them to keep away (but other males may enter the web regardless in an attempt to mate), and to possibly minimize the chances of prey becoming entangled in the web, which would interrupt courtship and mating. There is much competition between males in a natural habitat, and generally the first male on the scene will always try to mate first, but that is not always the case. Male Latrodectus are known to cohabit and copulate with sated subadult females as well. Latrodectus is one of many genera that uses pheromones for reproductive purposes. Pheromones are emitted by both sexes, but primarily aid the males with locating females. Females have been shown to react to both male and other females’ pheromones.
-Crossbreeding does not work.
Indication sac is bad:
This is not always true, but many times if the eggs stick to the female (picture), and not the sac, the eggs are not good. A better indication is that the female will discard the sac (picture). Most female Latrodectus are protective of their sacs, and guard them zealously (pictures below). They will readily discard them if infertile or parasitized, and shortly after spiderlings emerge, though. Some species such as geometricus leave the sac in the retreat regardless of viability and content. Sometimes the eggs will fall out before the female can secure them.
Defense of eggsacs:
Female NA widows love their sacs and are very protective of them, especially hesperus and mactans. But, geometricus differ from the others in that they rarely to never defend their sacs or themselves.
Great photos of defense of eggsacs
Sac production, most sacs produced:
Hesperus: 21
Mactans: 15
Variolus: 6
Geometricus: 33
Bishopi: 8
Eggsac shape and color:
Hesperus: tan and pyriform. Pear-shaped, and often somewhat spread at the top. About 13 or 14 mm in height and about 10 or 12 mm in diameter. Most often creamy yellow to light tan.
Mactans: gray and spherical. Almost always show the gray tinge and often are quite decidedly gray, even when freshly made. Also, they are more nearly spherical, about 11 or 12 mm in diameter, and with a conspicuous nipple at the top.
Variolus: gray and pyriform. Pear-shaped, and often somewhat spread at the top. About 13 or 14 mm in height and about 10 or 12 mm in diameter. Light tan with most often a tinge of gray.
Hesperus: photo
Mactans: photo
Variolus: photo
Bishopi: photo
Geometricus: photo
(eggsac photos are not mine)
Numbers of sacs and numbers per sac:
There are fewer egg sacs made by mactans than by hesperus. However, mactans lay more eggs per sac on the average, the mean for 185 sacs being 255 eggs, while for hesperus the mean of 464 sacs was 196 eggs.
Length of sperm retention:
Females can retain sperm for up to 600 days.
Maximum longevity, in days:
Hesperus: male- 369, male post-maturity- 196, female- 1049, female post-maturity- 952
Mactans: male- 235, male post-maturity- 127, female- 911, female post-maturity- 849
Variolus: male post-maturity- 155, female- 1063, female post-maturity- 822
Thanks for reading! Hope everybody learned something.