J. Benforado and K. H. Kistler.
Growth of the Orb Weaver, Araneus diadematus, and correlation with Web Measurements.
Psyche 80:90-100, 1973.

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GROWTH OF THE ORB WEAVER,
ARANEUS DIADEAfA TUS, AND
CORRELATION WITH WEB MEASUREMENTS*
BY JAY BENFORADO AND KENT H. KISTLER
Division of Research, North Carolina Department of Mental Health Raleigh, North Carolina 2761 I
It is a well-known fact that within any population of spiders of similar age there is considerable variation in the size of individual spiders of the same species. In literature as early as 1890, McCook has observed this variation and repeated observations (Comstock, I 940; Savory, 1928) have verified this phenomenon. Although ob- servations are frequent, explanations are few. Bristowe (1958) cites differences in feeding as a reason for differential size, but the reference is made merely in passing and to the authors' knowledge is not elaborated upon elsewhere. This paucity of explanation lends itself to further analysis of the factors contributing to the phenom- enon of differential size.
Our purpose in this paper is to isolate some of the factors which contribute to differential size in Araneus diadematus Clerck (for identification of species, see Levi, 1971 ), and to elaborate upon cer- tain of these factors as we are able.
'Corresponding with differential size, in an orb-weaver such as Araneus diadematus, differential growth is also manifested in chang- ing dimensions of the web. That large differences in dimensions exist betwen the individual webs of spiders is also a well-known fact. An attempt to clarify some of the factors influencing web changes is also made.
ENVIRONMENT:
The spiders used were from two cocoons of ha- neus diadematus, obtained from Canastota, New York, which hatched on April 26, 1972. From the time of hatching and throughout the experiment, the spiders were kept in a room which was lighted 16 hours per day aand kept cool during the eight dark hours by an air conditioner. (See Witt, 197 I ) .
EARLY REARING:
At the time of hatching, the offspring from each *Manuscript received by the editor March 1,1973



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19731 Ben forado &? Kistler - Araneus diadematus 91 cocoon were placed in a separate rearing box. The spiderlings were kept in these boxes, living on a communal web with a constant sup-. ply of loose gnats in the box, until they began to build individual webs approximately three weeks after hatching. As each animal built her first web she was removed from the rearing box and placed in an individual glass tube, approximately I X 7 cm, with the ends
of the tube stoppered with cotton. From the time the animals were placed in the tubes until onset of the experiment they were fed ap- proximately 10-15 gnats per week, by placing the gnats in the tube with the spiderling. The animals were watered by wetting the cot- ton with water daily.
DISTRIBUTION:
Seven weeks after hatching the two sets of spider- lings were each 'separated into three equal groups by means of a random numbers chart. No attempt was made to distribute males and females evenly. Although the growth (body weight) of males and females differs, it has been shown that the early growth of both sexes is alike (Witt et a!., 1972). Because of the short duration of the experiment and the difficulty in identifying male spiders before the last molt, distribution of males and females was left to chance. At the time of initial grouping the two sets of spiderlings num- bered twenty and thirty respectively. It was decided to feed each of the three groups of each set according to a different feeding sched- ule: one group every day, one group twice weekly, and one group every ten days. Thus there were six groups, one for each set of offspring on each feeding schedule. After one week of this procedure, however, it was decided because of the small size of the groups to reduce the number of schedules to two, and the middle schedule was dropped and its members distributed randomly between the lighter and heavier-fed groups.
Data for animals that died or escaped during the course of the experiment were removed, so figures represent only animals observed for the duration of the experiment.
WEIGHING: Each spider in the heavy-fed groups was weighed once a week, to O,.I mg, while animals in the light-fed groups were weighed on the day of feeding and the day after feeding. WEB ANALYSIS: After eleven days of controlled differential feeding in the tubes, the spiders were transferred to aluminum and glass laboratory cages, 50 X 50 X 10 cm. At this time the animals were eight weeks old. From this time on the spiders began to build webs. Photographs of webs were taken daily and analyzed (Reed et al., 1965). Daily records of web building were kept and the webs were



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92
rag
80
60
40
20
0
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light fed
heavy fed
JUNE J ULY
Figure 1. Mean weights of 19 light-fed and 15 heavy-fed baneus diadematus with standard errors (vertical lines). Figures are for b,oth sets of spiderlings combined. Sharp increases in weight in the light-fed group are due to the animals being weighed before and after feeding. Note the increasing difference in weight between the light-fed and heavy- fed group.




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19731 Benf orado &f Kistler - Araneus diadematus 9 3 destroyed daily with the thread left in the cage for the spider to digest.
FEEDING:
While in the tubes, the spiders were fed by placing a previously weighed de-winged housefly in the tube daily or every ten days. Those spiders that would not eat a housefly had three to seven unweighed gnats placed in their tube. By visual inspection the following day it was determined whether the fly had been eaten. The remains of the eaten flies were then weighed to obtain an approxi- mation of the amount eaten by each spider. The spiders were watered by wetting the cotton every other day. After being placed in the cages, if the spider had a web, feeding was by means of placing the housefly in the web; if there was no web, we attempted to induce the animal to eat by placing the fly in front of its mouth. The heavy-fed spiders were offered at least one fly per day and more, if they would accept it. The light-fed group was fed one fly once every ten days. If on the day of feeding of the light-fed group a spider would not eat, a note was made and the attempt repeated until successful. All spiders were watered on Mondays, Wednesdays and Fridays by spraying a small amount of water in each cage.
MOLTS: From the onset of the experiment molts were recorded by date of the molt to give an indication of the maturation of the animal.
FEEDING AND WEIGHT INCREASE: At the end of a period of five weeks the two feeding schedules resulted in two significantly differ- ent weight groups. This development is shown in Figure I, which illustrates the increasing difference in weight between the two groups. At the onset of differential feeding the mean weights of the two groups were alike, however, a T-test between the mean weights at the 'end of the experiment is significant at the O. I %> level. An analysis of covariance was performed on the data. Because the original data was non-homogeneous, a transformation [log (x+ 10) ] was made (Wines, 1962). The initial observation was used as a covariate in the analysis of covariance. Because the analysis of covariance indicated no significant difference in the behavior (growth) of the two families, all figures are for both families com- bined. For the heavy-fed group the mean weight changed from 7.93 mg k 1.04 on June 12 to 74.28 mg å
10.93 on July 17. The
mean weight of the light-fed group changed from 6.40 mg å 0.98



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94 Psyche [March-June
on June 12 to 17.91 mg å 2.56 on July 13 ; there was a significant interaction between time and feeding schedule below the 1% level. FEEDING AND MATURATION: If the number of molts over time is taken as an indication of speed of maturation, then a relationship between feeding and rate of maturation can be seen. During the period of differential feeding the number of molts of the heavy-fed and light-fed groups differed significantly at the 5% level. The heavy-fed group had a mean number of 3.0 molts while the light-fed group had a mean number of 2.3 molts. These results are in agree- ment with the findings reported by Deevey (1949) with Latrodec- tus mactans (Fabricius) and indicate that in the laboratory with only food quantity as a variable, a relationship exists between the rate of weight increase and the rate of maturation. INITIAL WEIGHT AND RATE OF GROWTH: From the beginning of the experiment we noted a wide variation of weights of the individ- ual animals. At the onset of differencial feeding individual weights ranged from 1.1 mg to 16.2 mg. In both the light-fed and heavy- fed groups there existed a positive correlation between initial weight and final weight. For the light-fed group r = 0.7713 and for the TABLE I
Light-fed Heavy-f ed
Measurement Early Late Early Late
Mean wt. (mg) of spiders 12.52 22.30 20.14 59-34 Spiral area (cm2) 118.92 119.51 I 19.88 138.32 Center area (mm2) 7 I I .OO 877.53 920.30 1424.30 Thread length (m) 7-35 7-47 7-56 8.47
Mesh width (mm2) 20.16 22.34 21.79 27.48 ----------------------.--..--------.--.--------.-----------------.-----.-----------.-----.----- Angle regularity 4.25 4.16 5.52 4.62
# of oversized angles 1.67 1.87 2.50 I .80 Relative deviation of
spiral turns (South) 0.34 0.33 0.4 I 0.35 Selected measurements of webs built by a group of light-fed and heavy-fed spiders. Because not all spiders built on the same day, early and late webs of both groups were chosen from two five day periods two weeks apart. Measurements are divided into those which measure size (above the broken line) and those which measure regularity. Note the difference between the light-fed and heavy-fed animals in measures of web size at the late date. While the heavy-fed group increased in all size measures (see Fig. 2), no web regularity measures changed. For an explanation of web measure- ments see Witt et al., A Spider's Web.




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19731 %enf orado ^f Kistler - Araneus diadematus 9 5 Figure 2. Selected web samples from two spiders: one heavily-fed spi- der and one light-fed spider. Webs are from the periods measured in Table 1 and are all reproduced to the same scale. Note that while both the heavy-fed and light-fed animals began with webs of similar size, after two weeks of differential feeding the large, heavy-fed spiders' webs had increased in size while the webs of the small, light-fed spiders remained the same size.




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heavy-fed group r = 0.9111 ; both correlations are significant at the 0.1% level. In most instances those animals with the extreme weights at the beginning of feeding remained the extremes in their group. Reasons for the variation in initial weight are unknown. Different rates of growth for light and heavy hatchlings have re- cently been shown to occur in several species of spiders, apparently independent of food available, and appear correlated with different lengths of life (Reed and Witt, 1972).
AMOUNT EATEN: An approximation of the amount eaten was ob- tained
for a three week period. For nine heavy-fed animals the mean amount eaten during the three week period was I 15.8 mg and for fifteen light-fed animals the mean amount for the same period was 35.0 mg. Within each group, however, there was an enormous variation in the amount consumed: in the heavy-fed group the amount eaten by individual animals ranged from 209.0 mg to 42.6 mg while in the light-fed group the amount eaten ranged from 49.6 mg to 3 gnats weighing I 8 mg.
FEEDING AND WEB CHANGES:
Table I gives a summary of web
changes that accompanied the growth of the animals. In measure-
ments of web size both groups increased, with the heavy-fed group having a much larger increase as illustrated in Figure 2. In measure- ments of web regularity both heavy-fed and light-fed groups re- mained constant, as shown in figures of Table I. The observed differential
growth and development in Araneus
diadematus seems to be a function of several factors. Although an exposure to a greater than normal supply of food generally results in faster than normal growth and development, even within a group exposed to the same food supply there seems to be a great variation in growth rates. Evidence of these differences is expressed in the increasing standard errors in Figure I, and seems to be dependent upon individual factors in the animals rather than environmental variations. Large differences in the amount eaten by individual animals in the laboratory existed and presumably exist in nature. These differences seem to correspond to differences in the rate of growth in agreement with the findings of Turnbull in other species of spiders (Turnbull, 1960, 1965). However, whether these dif- ferences in the amount of food eaten are due to differences in pro- ficiency in prey-catching or to differences in appetite or some other factor in the animal is not clarified by our findings.



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19731 Benforado å£ Kistler - Araneus diadematus 97 Another important factor influencing differential growth is the initial weight of the animal. Variations in initial weights within a family are generally retained during the course of development. Al- though several possible reasons for different initial weights within a family have been given by others, the authors are reluctant to offer any explanations.
In an orb web weaving spider such as Arfineus diadematus the amount of food available to the animal is roughly equivalent to the number of prey which become entrapped in the web. The number of prey entrapped in the web is in turn determined by a number of variables such as web-site, size and fine structure of the web, and frequency of web building. Thus, it can be seen that the interaction of the variables resulting in differential size and growth is complex and can be divided into those factors which influence the amount of food available TO the spider and those factors which influence the spider's use of the food available to it. Repeated attempts have been made to explain web characteristics in terms of characteristics of the individual spider (Peters, 1936). More common, however, has been the notation of changes in the form of the orb web during the life of the spider (Tilquin, 1942; Savory, 1952) and the attempt EO relate these changes to changes in the animal ( Witt and Baum, 1960; Witt, 1963 ; Reed et d., 1969). Because influencing factors vary concurrently, it is frequently diffi- cult to assess the causes of changes in the form of the web. In our experiment we attempted to isolate the effect of one vari- able (weight) while minimizing the effect of a variable which nor- mally changes concurrently (time). All animals used hatched on the same date, however, one group (the heavy-fed) gained consid- erable weight over the period measured. The web changes accom- panying these weight increases are summarized in Table I. Be- cause all of the animals were hatched on the same date. we conclude that increases in web size are due to differences in size of the ani- mals resulting from differential feeding rather than differences in age. If appetite were a factor influencing web size, it would appear that the hungrier, light-fed animals would build a larger web in an attempt to catch more food; however, this is not the case. The relationship between food and the web of a spider is a deli- cate one. Without food, the spider's web-building ability diminishes, but without a web there is no food ( PeakaII, 1968). Thus, like a businessman, the spider faces the law of diminishing returns. It appears that the hungry spider chooses to conserve its resources



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98 Psyche [March-June
rather than gamble on a larger web trapping more food. Early food deprivation experiments (Witt, 1963) show that the spider con- tinues to build the same size web when deprived of food, but with less thread until finally a decreasing in web size occurs. Because our hungry (light-fed) animals were kept on a diet closer to a main- tenance level than a deprivation level, we observed no decreases in web dimensions.
Feeding conditions in a natural environment vary more than those imposed in a controlled laboratory. Yet the spider is able to survive in these naturally diverse conditions because of its adaptability. In situations where there is little food available, the spider is able to survive by growing at a slow rate and maintaining the same size web. Where food is abundant, the spider takes advanta,ge of the situation, growing at a fast rate and increasing the size of its web. The spider has developed a method for coping with a wide range of feeding conditions. By varying its body and web growth, the spider can survive under the diverse conditions imposed by nature, thus minimizing the necessity of seeking new food supplies .and re- locating the web. Our findings provide new insight into the spider as an example of an animal that adapts itself successfully to its en- vironment.
Spiders from two cocoons of Araneus diadematus were exposed to five weeks of two different feeding schedules: one group was of- fered large amounts (one housefly per day) of food, the other group scarce (one fly every ten days) amounts. Although both groups in- creased in weight, weight gains of the heavy-fed group were signifi- cantly greater than those of the light-fed group, regardless of cocoon origin. Within each group there was a wide variation in the growth of individual animals, indicating the presence of factors other than food supply; i.e. animals with extreme weights within a group at the onset remained the extremes.
In conjunction with increases in weight, over the three week period of observation, webs of the heavy-fed spiders showed an increase in size but not in regularity and shape in comparison to webs of the smaller, light-fed animals of the same age which did not change. Such data suggest an increased chance of survival of the species through variations in rate of growth and maturation dependent on environmental factors.




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19731
Ben f orado 3 Kistler - Araneus diadematus This work was carried out in the laboratories of the North Caro- lina Department of Mental Health and was supported by Grant Number GB 25274 from the National Science Foundation to Peter N. Witt. The authors gratefully acknowledge the assistance of Dr. Peter N. Witt during all stages and the assistance of Mrs. Mabel B. Scarboro during the period of laboratory work. BRISTOWE, W. S.
1958. The World of Spiders. Collins, London. COMSTOCK, J. H.
1940. The Spider Book. Revised and edited by W. J. Gertsch. Com- stock, Ithaca, N.Y.
DEWEY, G. B.
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1971. The Diadematus group of the orb-weaver genus Araneus north of Mexico (Araneae: Araneidae). Bull. Mus. Comp. Zool., 141 : 131-179.
MCCOOK, H. C.
1890. American Spiders and Their Spinningwork. Vol. 2, Published by the author, Philadelphia.
PEAKALL, D. B.
1968. The spider's dilemma. New Scientist, pp. 28-29. PETERS, H. M.
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TURNBULL, A. L.
1965. Effects of prey abundance on the development of the spider Agelenopsis potteri (Blackwell) (Araneae: Agelenidae). Canad. Ent. 97: 141-147.
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