Although fatalities from spider bites are rare, there are certainly many insects and other small animals that experience how serious a spider bite is, and how venom can severely destabilize their body functions before they are digested.
But how do spiders actually optimize their use of venom? How much venom they have in their glands is affected by how long time ago they used their venom to kill another prey.
Often, a spider will capture more than one prey in a day. However, as it can take some time for a spider to regenerate lost venom, it is likely that spiders have some sort of strategy for how much venom they actually inject into caught prey, and which prey they prefer in relation to their venom status.
Previously it has been reported (Robinson, 1969)1 that some spiders either use a so-called long bite or a short bite, and that short bites were for smaller insects and longer bites for larger animals. The investigation also hypothesized that longer bites delivered a larger dose of venom than the shorter bites. In another study, were they did not estimate the dosage of venom injected, it was hypothesized that the amount venom depended on how difficult it was to handle the prey - basically, a way of defense against being damaged by the prey during a fight.
In 1998 it was discovered (Malli et. al 1998)2 that the species C. salei injected more venom than would seem necessary from LD50 numbers in cases where the prey was difficult to handle.
In other studies (Wullschleger and Nentwig, 2002)3 the strategy has been to remove venom from the spiders to see if their reduced capability of delivering a high dose of venom, would change their choice of prey.
In Wullschleger and Nentwig (2002) it was also shown that C. salei indeed acted differently when their venom glands were depleted.
Humans are definitely not considered a prey by spiders, and most, if not all, cases of spider bites on humans are delivered as a part of a defense. The fact that definitely not all spider bites are dry shows that venom is injected into potential attackers also as a mean of defense, and not only as means of preparing the prey and or improving digestibility.
This study also suggested that the venomous spiders anticipate the sensitivity of the prey to venom and adjust the venom dose accordingly.
Spiders have always had a bad reputation, and have been blamed for many human sufferings throughout centuries. However spiders are beneficial to man and only very few species causes damaging envenomations. In the US, only bites from Brown Recluse spiders, Brown Widow Spiders and Black widow spiders can cause severe envenomation. Luckily, most bites from these two (three) spiders goes unnoticed.
Envenomation from brown recluse spiders results in Loxoscelism, a medical condition that causes skin ulcerations. There are more than 100 species in the genus Loxosceles, and most are located in South America. Basically, they are quite peaceful and most bites occur when they are trapped - such as in a shoe or when someone rolls over them in bed.
In Brazil, bites from Brown Recluse spiders are claimed to be the third most frequent cause of envenomations by animals. Antivenom is often tried, however, the antivenom is administered so late that its effect is negligible. Therefore, there are still doubts that antivenom should be used against brown recluse spider bites. However, a study (Pauli et al. 2006)4 claims that even delayed administration of antivenom can reduce the required healing time and diminish the systemic damages imposed by the effects of Loxoscelism. Brown Recluse bites are also very common in Argentina, Chile and Bolivia.
Recently, a family in Kansas was found to have been living with thousands of brown recluse spiders5, not a single bite occurred. Such findings indicate, that even in places with high densities, bites are rare, and that reports about bites in areas where the brown recluse spider is not endemic, should be taken with a grain of salt.
This picture shows how a brown recluse spider bite might look after a couple of days. Please notice that this is a severe case of Loxoscelism.
Basically, Black Widow spiders can be divided into two groups: mactans and geometricus. That is, the Black Widows (mactans), and the Brown and Grey Widows (geometricus).
Widow spiders like to stay in dark and dry habitats. Therefore, most bites occur because a Widow spider has taken residence in a dark, dry place where it suddenly gets pressed towards flesh without any chance of escaping.
In their natural habitats, the Black Widow spider finds shelter beneath rocks, in smaller burrows and other dark places. Almost all bites are caused by female Black Widows.
Please notice that the notion that female Black Widows eat males following mating is slightly wrong. It is something that has been observed in close containers only, where the male could not escape.
Only a few studies have investigated medical cases of Latrodectism. However, a few characteristics stand out. Latrodectism is characterized by local pain radiating out from the bite site. Sometimes, the person bit can experience several neurological effects such as diaphoresis.
The sensitivity to venom from Black Widows varies. Some patients are only slightly affected while others are close to dying.
Bites are also associated with chest pain and muscle spasms. However, the first symptom is acute pain at the site of the bite, which usually starts within 15 to 60 minutes. Usually the symptoms levels off within a few hours or days.
Due to the risk of allergic reactions, there is a general reluctance against using antivenom.
Bites from Brown Widows are less severe than bites from Black Widows, and the symptoms are minor.
Due to the risk of allergic reactions, there is a general reluctance against using antivenom in cases that are not severe.
The Australian Funnel-Web spider is rather dangerous, especially if you only look at what doses it is capable of delivering and how much is needed to kill an animal. In eastern Australia there are 5-10 severe envenomations each year. Most often, nothing happens after a bite, but in some cases the heart will beat faster (tachycardia) or the heart will beat really slow (bradycardia). Both conditions are dangerous. Severe cases needs treatment to avoid fatalities. Luckily, antivenom is available and effective.
Armed spiders are Phoneutria spp. species. The Brazilian Wandering spider is an armed spider. Sometimes these spiders are imported to USA with bananas. Most imported specimens are harmless though.
There is a spider, the Woodlouse spider, which is fairly common in homes and gardens in USA. Although its bite is inoffensive and not medically important, it can cause pain for a short while. It is mentioned here because it is a common spider with relatively large fangs it displays when threatened. Secondly, it is often misidentified as the Brown Recluse spider even though Woodlouse spiders look very different. A study in the early nineties stated erroneously that the bites from Woodlouse spiders had medical importance, but a review by Vetter and Isbister (2006)6 showed that the mean duration of pain was 40 minutes on average.
In 1990, a study7 found that bites from Wolf spiders were relatively harmless. Before then antivenom was used to treat Wolf spider bites. Today, antivenom is not used to treat bites from Wolf spiders.
Yellow Sac spiders have long been known in the medical literature to be the causative agent of serious envenomations. However, later studies have shown that bites from Yellow Sac spiders are rather benign, something in the range of that of bee stings. Recently Yellow Sac spiders were found to invade Mazda 6 automobile engines.
Below are short descriptions of some of the spiders featured at seriousspiderbites.com. Please notice that most often it is females that inflict bites on humans. This is simply due to the fact that spider females are often larger than male spiders.
In contrast to common belief, the Brown Recluse spider is neither found in Florida nor in California. However, it is possible to find plenty of Recluse spiders in states such as Texas and Arizona. The venomocity of brown recluse spiders is somewhat overstated, as in most cases, luckily, recluse bites are not severe.
Read more about brown recluse spiders.
Brown Widow spiders (Grey widows is a name often encountered too) are the least dangerous of the Widow spiders because they are rather small. In the US, Brown Widow spiders are spreading along the East coast, and are good at accommodating to the North American climate. The symptoms of Brown Widow spider bites disappear within a few hours. According to Wikipedia the species has been discovered independently in USA and in Africa. Basically, there is no reason to panic if you encounter one of these creatures. It definitely won't be able to kill you.
Latrodectism is the condition following bites from Black Widow spider. The Latin name of this spider is Latrodectus - the most common Black Widow being the mactans. Many females have a red hourglass on their abdomen. Some only have a couple of red spots. In contrast to Brown Widows, Black Widow spiders (especially females) are seriously dangerous to humans. Females haves relatively large venom glands and can deliver a bite that ultimately can cause cardiac arrest. Although fatalities are extremely rare, one should seek medical counseling if bitten by a Black Widow.
Hunting spiders (Cupiennius spp.) are very common in pet stores. They are very fast and are also known as a Wandering spider. They are nocturnal, so in their natural habitat they will wander around on the forest floor foraging.
1. Robinson M.H. Predatory behaviour of Argiope
argentata (Fabricius). American Zoology 9, pp. 161-173 (1969)
2. Malli H., Imboden H. & Kuhn-Nentwig, L. Quantifying the venom dose of the spider Cupiennius salei using monoclonal antibodies Toxicon 36, pp. 1959-1969 (1998)
3. Wullschleger B., Nentwig W. Influence of venom availability on a spider's prey-choice behaviour Functional Ecology 16(6), pp. 802-807 (2002)
4. Paulia I.B., Pukac J., Gubertd I.D., Minozzo J.C The efficacy of antivenom in loxoscelism treatment Toxicon 48 123-137 (2006)
5. Vetter R.S., Barger D.K. An infestation of 2055 brown recluse spiders (Araneae: Sicariidae) and no envenomations in a Kansas home: implications for bite diagnoses in nonendemic areas J. Med. Entomol. (39) pp. 948-51 (2002)
6. Vetter RS & Isbister, G.K. Verified bites by the woodlouse spider, Dysdera crocata Toxicon 47 826-829 (2006)
7. Ribeiro LA, Jorge MT, Piesco RV, Nishioka SA. Wolf spider bites in Sao Paulo, Brazil: a clinical and epidemiological study of 515 cases. Toxicon (28) pp. 715-717 (1990)
Thank to Manticor and Pooair for allowing me to use their photos.
Your spider site here.