humpbacked orbweaver

(Eustala anastera)

Conservation Status
humpbacked orbweaver
Photo by Alfredo Colon
  IUCN Red List

not listed

     
  NatureServe

NNR - Unranked

     
  Minnesota

not listed

     
           
           
           
           
           
           
           
 
Description
 
 

Humpbacked orbweaver is a small typical orbweaver. It occurs in the United States from Maine to North Dakota south to Florida and California, in southern Quebec and Ontario Canada, and in Central America. It is very common in the east, less common in Minnesota. It is found in open woodlands and woodland edges on trees and shrubs. During the day it rests on a dead twig or branch, where its color and markings camouflage it well, making it difficult to see.

The female is 316 to (5.4 to 10 mm) in length and has a to ¾ (10 to 20 mm) legspan. The male is slightly smaller, to (3.9 to 9.5 mm) in length.

The front part of the body (cephalothorax) is much smaller than the abdomen. The upper side (carapace) is mostly brown but dark on the sides. The thoracic region has a deep longitudinal furrow in the middle and is covered with long white hairs and with shorter downy hairs.

There are eight eyes arranged in two parallel rows of four eyes each. The eyes are not on raised projections (tubercles). The rear row is slightly curved forward, the front row is straight or slightly curved backward. All of the eyes are small, but the median eyes are larger than the lateral eyes, and the posterior median eyes (PMEs) are slightly smaller than the anterior median eyes (AMEs). On each side the lateral eyes are widely separated from the middle (median) eyes and are almost touching each other. The median ocular area (MOA), the area defined by the middle four eyes, is longer than wide and narrower in front than behind. There is a often a dark band on the face between the anterior lateral eyes (ALEs) and the AMEs, and this band is sometimes continuous across the face. There is sometimes also a dark band between the PMEs.

The abdomen is large, longer than wide, and triangular when viewed from above, widest in front and pointed at the rear. When viewed from the side it appears squarely cut off (truncate) with a distinct hump at the end above the spinnerets. This is the feature that gives the species the first part of its common name. Some females have two or three humps in a line. There are no humps in the shoulder area (humeral tubercles). The color and markings on the upper side is variable but follows one of three basic patterns. It is most often gray or light brown with a dark, scalloped, triangular mark (folium) stretching from the front to the rear. The folium is sometimes faint. Some individuals have a bold dark stripe in the middle of the abdomen with the folium faint or missing. Some individuals have a white abdomen and a black folium with either three large black spots on the front margin or a continuous irregular black band on the front margin. The underside of the abdomen has a large black patch surrounding a white spot.

The legs are spiny and mostly gray or light brown with irregular dark bands. The first pair of legs is the longest, the third pair is the shortest. On the male, the first segment (coxa) on the front pair of legs has a hook-like spur on the underside. This spur fits into a groove on the upper side of the third segment (femur) on the second pair of legs.

 
     
 

Size

 
 

Female Body Length: 316 to (5.4 to 10 mm)

Male Body Length: to (3.9 to 9.5 mm)

Legspan: to ¾ (10 to 20 mm)

 
     
 

Web

 
 

A loose web with few threads is often constructed on a dead branch away from leaves or on a tree trunk. It is called an “orb”, which gives this family of spiders its common name. The orb is (19 to 30 cm) in diameter and has 18 to 21 radii and 58 to 64 sticky (viscid) threads. Webs of juveniles are smaller and have more threads. The web hangs more or less vertically and is somewhat asymmetrical, largest on the lower half. It is an open web with a medium-sized hole in the center. There is no retreat.

 
     
 

Similar Species

 
     
     
 
Habitat
 
 

 

 
     
 
Biology
 
 

Season

 
 

 

 
     
 

Behavior

 
 

Adults are active at night and rest on a dead branch during the day. The web is constructed in the evening after dark and is torn down in the morning, usually before daylight. When resting it usually keeps its legs slightly spread.

 
     
 

Life Cycle

 
 

The eggs are hung on a radius of the web.

Spiderlings in the final development stage (instar) overwinter.

 
     
 

Food

 
 

 

 
     
 
Distribution
 
 

Distribution Map

 

Sources

24, 29, 30, 82.

 
  1/11/2022      
         
 

Occurrence

 
 

 

 
         
 
Taxonomy
 
  Class Arachnida (arachnids)  
 

Order

Araneae (spiders)  
 

Suborder

Araneomorphae (typical spiders)  
  Infraorder Entelegynae (entelegyne spiders)  
  Superfamily Araneoidea (araneoid spiders)  
 

Family

Araneidae (orb weavers)  
 

Subfamily

Araneinae (typical orbweavers)  
 

Genus

Eustala  
       
 

Synonyms

 
 

 

 
       
 

Common Names

 
 

humpbacked orbweaver

 

 

 

 

 

 

 

 

 

Glossary

Carapace

The hard, upper (dorsal), shell-like covering (exoskeleton) of the body or at least the thorax of many arthropods and of turtles and tortoises. On crustaceans, it covers the cephalothorax. On spiders, the top of the cephalothorax made from a series of fused sclerites.

 

Cephalothorax

The front part of a spider’s body, composed of the head region and the thoracic area fused together. Eyes, legs, and antennae are attached to this part.

 

Femur

On insects and arachnids, the third, largest, most robust segment of the leg, coming immediately before the tibia. On humans, the thigh bone.

 

Folium

On some spiders, the leaf-shaped marking on the upper side of the abdomen.

 

Instar

The developmental stage of arthropods between each molt; in insects, the developmental stage of the larvae or nymph.

 

Tibia

The fourth segment of an insect leg, after the femur and before the tarsus (foot). The fifth segment of a spider leg or palp.

 

Tubercle

On plants and animals: a small, rounded, raised projection on the surface. On insects and spiders: a low, small, usually rounded, knob-like projection. On slugs: raised areas of skin between grooves covering the body.

 

 

 

 

 
 
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Alfredo Colon

 
    humpbacked orbweaver   humpbacked orbweaver  
           
 
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  Alfredo Colon
8/6/2019

Location: Woodbury, MN

humpbacked orbweaver  
  Alfredo Colon
8/2/2019

Location: Woodbury, MN

humpbacked orbweaver  
           
 
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Created: 1/11/2022

Last Updated:

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