Spiders at the Cinema

ACADEMIA Spider ecology, behavior, and learning Behavioral Ecology Spiders at the Cinema Focus on extremely receptive sensory organs allow MACIEJ BARTOS them to perceive those signals accurately, There are plenty Faculty of Biology and Environmental Protection helping them construct complex webs and of spiders that are University of Łódź remotely localize insects flying or scuttling as colorful as the [email protected] nearby. Eyesight and sensory organs used prettiest butterflies, Dr Maciej Bartos studies the behavior of jumping spiders to perceive chemical stimuli also play an have better eyesight important role in orientation and commu- than many birds or In spite of severe brain size limitations, nication between individuals. Most spiders’ mammals, and exhibit many spiders are able to solve simple eyes, like those of insects, are not more complex behavior complex problems, recognize their prey capable of perceiving complex images. They than many higher allow them to notice motion, changes in light animals. Pictured: with great accuracy, and learn new intensity, and to a lesser degree allow them American Cardinal hunting techniques to identify a partner or prey. As such the jumper (Phiddippus majority of spiders exhibit relatively simple cardinalis) To most people, spiders are hairy crea- behavior. However, not tures that lurk in dark corners behind the all spiders are limited by cupboard. They are widely disliked, and low acuity of vision. in some people even trigger panic fuelled There is in fact a large by tales of the creatures’ terrible venom. group of diurnal spiders However, arachnophobia can be cured: those whose world is domi- afflicted simply need to find out more about nated by visual stimuli. spiders and get to know them better. In They see color, includ- reality, apart from the genuinely scary ones, ing wavelengths in the many spiders are as colorful as the prettiest UV range, they are able butterflies, have better eyesight than many to differentiate complex birds or mammals, and exhibit more com- configurations of viewed plex behavior than many higher animals. objects, and their visual Their excellent vision and small brain size acuity exceeds that of make them a good model for research into learning and decision making processes in animals. Perhaps surprisingly, some such studies involve setting up miniature cin- emas for the spiders, allowing researchers to study how they perceive their world and how they make decisions based on the information available. In light and darkness Spiders form a large and diverse group of predatory arthropods. The majority of the over 42,000 described spider species have poor eyesight and are mainly active at night. The sensory world they live in is made up of different kinds of vibrations: airborne, those transmitted by the substrate, their nests or webs, and tensions in their cuticle trig- gered by various mechanical stimuli. Their Keith Stewart/bugsinthenews.info 16 No. 2 (30) No. 2011 ACADEMIA Focus on Behavioral Ecology Maciej Bartos many vertebrates, even ones with vision the corneal lens there is a long tube reach- Dune jumping spider as good as cats or pigeons. This is the fam- ing far into the cephalothorax. At the end is examining its prey ily of jumping spiders (Salticidae), owing the retina, with another lens in front, magni- their name to their ability to jump to escape fying the image 1.5 times. The long, tubular enemies or while hunting prey. They are eyes, the narrow retina and the additional the largest and most diverse spider fam- lens mean that the spiders are as if looking ily. Although only 60 species of jumping through a telescope – the image is magni- spiders can be found in Poland, there are fied, although the field of vision is very nar- over 5300 different species worldwide. It row (approx. 10o). Despite this and the fact is worth noting that the majority have been that the corneal lens is immobile, making it described by Polish taxonomists, Prof. Jerzy impossible to look around, jumping spiders Prószyński, Prof. Wanda Wesołowska, and have developed a unique muscular system Prof. Marek Żabka. moving the retina inside the cephalothorax while the lens stays fixed. The system of six Extraordinary eyes muscle pairs attached to the surface of the Despite their name, the most extraordi- visual tube provides the retina with relative- nary thing about jumping spiders is not ac- ly free movement inside the cephalothorax, tually their ability to jump – which is, after increasing the visual field to around 60o. In all, shared by certain other spiders. They are order to see an object in full, the spider must truly distinguished by their extremely well first examine it closely – scan it bit by bit – developed vision, centered in one of their and assemble the images to form a whole. four pairs of eyes, which represent a unique The retina has a distinct, narrow V shape of evolutionary adaptation in the animal king- densely spaced receptors, with the arms of dom in terms of structure and function. The the V facing towards the lens. This arrange- eyes are all located around the cephalotho- ment provides sharp resolution of objects at rax, giving the animal an almost 360o visual various distances from the observer, even field. Three pairs are secondary eyes; like without adjusting the eye. The retina is built the eyes of most insects, they are only capa- of four layers of cells; each layer is sensitive ble of perceiving changes in light intensity to a different light wavelength, enabling and movement, and they do not allow the jumping spiders to see color. spider to recognize complex patterns. This ability is provided by their principal eyes Huge eyes, small brain situated at the front of the cephalothorax. Jumping spiders have an extremely good When an object appears in the visual field, visual acuity and exhibit highly complex the spider first notices its movement using behavior, even though they have very small the secondary eyes; it then turns round to brains (no larger than a pinhead). Such a face it and examine it more closely using small brain size severely limits computa- the principal eyes. The eyes have several tional ability and, as a consequence, the important structural features differentiating animal’s behavioral complexity. It should them from secondary eyes and making them be remembered that neurons in inverte- extremely effective visual organs. Behind brates are larger than in vertebrates. There 17 2 (30) No. 2011 ACADEMIA Spider ecology, behavior, and learning is huge variation in cognitive abilities of also able to learn new hunting methods. different groups of vertebrates, even just Portia specializes in hunting a very danger- Behavioral Ecology among mammals; comparing their brain ous prey – other spiders – often “imperson- sizes and structures to those of jumping spi- ate” sexual partners of the host of the web ders makes it clear how limited the latter’s they are entering, sending signals interpret- Focus on “wiring” must be. But paradoxically, the ed by the host as encouragement for mating. behavior of jumping spiders – among the The host is usually fooled and approaches smallest in the order – is undoubtedly the the intruder unawares, allowing itself to be most complex of all spiders, marking them caught at a suitable moment. However, this out from other invertebrates. allows Portia to capture only certain spiders All invertebrates, spiders included, have using innate hunting strategies. Since each until recently been regarded as simple, pre- species has its own distinctive set of mating programmed animals that show no behav- signals, on entering the web of an unknown ioral flexibility or ability to learn. However, prey Portia uses trial and error to deter- this view – persisting since Cartesian times mine the correct code – sending out a wide – has gradually changed as our knowledge range of vibrations of varying frequency and research methodology improved. Under and intensity, selecting those that elicit a closer scrutiny it turned out that certain so- response from the host. They frequently cial hymenoptera are able to communicate approach the host spider, although in order with each other to transmit diverse, often to remain unnoticed they move only during complex information such as the relative gusts of wind preventing them from being location and distance between the nest detected. If there is no wind, they twitch the and food source. Although bees and ants web themselves to imitate air motion. They have been studied for many years, it is only create this cover to allow them to approach recently that researchers have started to their prey undetected. use contemporary methodology and cre- These small spiders’ cognitive ability is ative experimental settings in which the well demonstrated in situations when Portia animals are set problem-solving tasks or sees its potential prey, but is unable to reach are required to make a decision under con- it directly. In this scenario it uses the spatial trolled conditions, allowing scientists to see arrangement of its surroundings to plan and invertebrates in a new light: as animals with prepare a route and select the one which a good ability to use information available to will eventually lead it to the prey. This hap- them and solve complex tasks. pens even when the selected route leads in the opposite direction to the prey, and the object of the assault temporarily disappears Research carried out from Portia’s visual field. The behavior de- in recent years has scribed here requires the spider to create brought significant a mental cognitive map, a skill which until progress in our recently had been thought to only be pres- understanding of the ent in higher vertebrates. Analyzing Portia’s complexity of spider behavior, we are faced with the question: behavior.

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