The Functioning Of Bird Dropping Masquerading By A Crab Spider:Predator Avoidance And Prey Attraction

Masquerading organisms evolve to closely resemble inedible and generally inanimate objects such as stones,bird droppings,twigs and leaves to deceive predators or prey.These masters of disguise gain protection from predation or gain access to prey by being misclassified either as inedible objects by their predators,or as innocuous objects by their prey.While strikingly resembling sticks and leaves is the most common masquerades,bird-dropping masquerading is also well known in several groups of spiders.Phrynarachne is a genus of sit-and-wait predators in crab spiders(Araneae:Thomisidae).Their shape,size and colour look like bird droppings,and Phrynarachne(formerly Ornithoscatoides)ceylonica is often cited as a textbook example of bird dropping masquerade.It has long been assumed that P.ceylonica visually resemble bird droppings to avoid predation or gain access to prey.These two alternative hypotheses have never been empirically tested yet they continue to permeate literature and are often assumed to be an established fact.In addition,P.ceylonica often sit on bird droppings;furthermore,they also smell like bird droppings.Thus,it is conceivable that their volatile odour may also be used to attract prey that are similar to those being attracted to bird droppings.Yet this hypothesis also remains untested.In this dissertation,I used P.ceylonica as a model system to test whether visual and chemical bird dropping masquerading exists in animal kingdom,and whether this visual and chemical masquerade functions to avoid predators or attract prey.In order to understand this bird dropping masquerading spider better,in Chapter 2,1 conducted a field study in the Xishuangbanna Tropical Botanic Garden(XTBG),Yunnan,China,to investigate whether the bird dropping crab spiders gain access to prey by attracting insects,particularly flies by being mistaken for bird droppings by comparing the types and abundance of prey attracted and captured by P.ceylonica with the prey attracted by bird droppings in the same habitats.Specifically,in this field study I attempted to determine the main prey types and estimated prey-capture rate to provide baseline data for further studies of functioning of bird-dropping masquerading in P.ceylonica.Results showed that similar to bird droppings,P.ceylonica attracted and captured mainly dipterans,hymenopterans,lepidopterans and spiders,and about 89%of prey captured by bird dropping crab spiders were dipterans,of which about 69%were flies from the family Agromyzidae.In terms of prey-capture rate,field surveys showed that a mean of 23 flies per hour per spider was captured by spiders during about 1-h daily survey for five days.This study suggests that bird dropping masquerading may increase foraging success of this ambushing predator by being mistaken for bird droppings by its prey,particularly by flies.To test whether bird dropping crab spiders visually resemble bird dropping,In Chapter 3,I applied colour modeling to compare P.ceylonica and bird dropping colouration from perspectives of an avian predator and a hymenopteran predator such as a wasp.I used two physiological visual models that incorporate the spectral sensitivities of avian and hymenopteran visual systems,generating predictions on whether predators such as birds and wasps can distinguish between colour patterns of bird dropping crab spiders and bird droppings.Results showed that predators may have difficulties to distinguish between P.ceylonica and bird dropping from chromatic vision and achromatic vision.To test both predator-avoidance and prey attraction hypotheses experimentally,in Chapter 4 I performed behavioural experiments to test whether the appearance and odour of P.ceylonica could visually and chemically repel and hoodwink their predators,and/or attract and lure their prey.I used P.ceylonica as visual source or odour source and the spider-eating jumping spider Portia sp.as predators to test whether P.ceylonica visually and chemically detect predators.Results showed that appearance and odour of P.ceylonica visually and chemically repelled Portia.Furthermore,I used P.ceylonica as odour source and houseflies as prey to confirm whether P.ceylonica could use their odour to attract prey as found in the field observations.I provided evidence that the odour of P.ceylonica was able to attract houseflies.Finally,in Chapter 5 I applied SPME-GC/MS to identify compounds present in volatile odour of P.ceylonica that might reply predator and/or attract prey.Multiple compounds were identified but their functions are needed to be tested in the future to determine which one compound or blends of compounds could be the repellent or attractant in future work.This study set an important precedent on the study of chemical masquerade in animals.