Hitching a Ride on a Giant Fly

Even for the tiniest of species, sexual selection provides advantages to the 'best equipped' males. For the pseudoscorpion, Semeiochernes armiger, new research has shown that male-male competition for dispersal sites, rather than female-choice, is a strong determinant of large sexual dimorphism. 

Tiny pseudoscorpion hitching a ride. Males attempt to mate with females whilst on the fly
Image credit: Jana Christophoryová

Living in the forests of Panama these tiny, wingless creatures are faced with a challenge when it comes to dispersal. Luckily for them, transportation is available in the form of the the giant timber fly. 

The female timber fly lays her eggs on the rotting wood of the Ficus tree (where S. armigers are also found). Her larvae then burrow their way into the wood where they remain for  5 - 12 months before re-emerging as adults and flying away. In order for S. armigers to disperse, they must hang onto the body, legs and wings of the fly as it emerges from its pupal case. Using another animal for dispersal is known as phoresy. 

Sexual dimorphism: female (left); male (centre) with large chela peg (indicated by arrow) and triangular palps; small male (right) with no chela peg and small triangular palps
Image credit: Zeh & Zeh, 2013

Labelled pseudoscorpion (not the same 
species) notice the palps and the chelae; 
these are regions affected by sexual 
selection. Overall body size was 

not different between males and females. 

Image credit: reprinted from the Canadian Journal of 
Arthropod Identification with permission of the 
photographer Christopher Buddle

Observations discovered that each of these fly "bore holes" was guarded by a single S. armigers male. Body measurements showed that males with the largest pedipalps, especially the chelae; were the most likely to control a bore hole. The pedipalps are required for feeding in both males and females, but males also use them for competition. Males with smaller pedipalps were unlikely to control bore holes, and have reduced mating opportunities. Sexual selection selects for males with bigger pedipalps as they will be the ones mating. 

Females wishing to disperse gather near the bore holes of soon-to-emerge flies. The male who controls the bore hole will mate with females whilst on the fly or when they reach their destination. As many as 22 pseudoscorpions were seen climbing aboard their fly host!  

Q:What benefit does transporting pseudoscorpions have for the fly? 

Sources:

• ZEH, J. A. & ZEH, D. W. 2013. On the threshold of dispersal: hitchhiking on a giant fly favours exaggerated male traits in a male-dimorphic pseudoscorpion. Biological Journal of the Linnean Society, 108, 509-520