|Female damselflies generally mate repeatedly and use their most recently received sperm for fertilization. During copulation, the accessory penes of males are capable of both sperm deposition and displacement. Males often maintain physical contact (tandem) with females following copulation to prevent other males from displacing their sperm from the female's sperm storage organs prior to her ovipositing.|
|COPULATION AND TANDEM GUARDING
Damselfly species in the genus Ischnura usually oviposit without an accompanying male, although some oviposit in tandem. This is enigmatic because sperm deposited in an unguarded female could be displaced by another male. I have accumulated evidence that, based on mating patterns and guarding strategies, 3 functional groups of species exist in this genus and that species within each group share many behaviors, life history patterns and morphologies. Group I includes species whose females mate only once and therefore are effectively monandrous, Group II includes polyandrous species whose males guard females in copula, and Group III are polyandrous species with tandem guarding males.
Data were aggregated from 17 ischnuran species for analysis (see Robinson, J. V. & R. Allgeyer. 1996. Covariation in life-history traits, demographics and behavior in ischnuran damselflies: the evolution of monandry. Biol. J. Linn. Soc. 58:85-98.) Group I species (followed in parentheses by our abbreviation for them) are: aurora (AUR), demorsa (DEM), hastata (HAS), kellicotti (KEL), perparva (PER), posita (POS), and verticalis (VER). Group II species are: cervula (C&E), damula (DAM), elegans (C&E), graellsii (GRA), erratica (ERR), pumilio (PUM), ramburi (RAM), and senegalensis (SEN). Group III species are: denticollis (DEN), and gemina (GEM). It is interesting to note that species within each group have much in common.
Polyandrous in copula guarding species are larger in size than species in the other groups. They also show less sexual size dimorphism. The geometric mean regression slope of logarithmically transformed hind wing lengths for males or females exceeds 1.0 (P<0.05).
|At maturity females of some ischnuran species develop a cryptic appearance due to pruinescence. On the left is a picture of a mature I. hastata female, on the right is an immature female. This cryptic change appears to be confined to monandrous species.|
In most ischnuran species mature females may be of any of 2 or more color forms (i. e. the species are polychromatic). Of the species considered only 4 are monochromatic; these 4 are all contained in the monandrous group.
The following figure portrays a typical ischnuran penis and female reproductive tract. The terminal flagella of the penis can always access the bursa copulatrix (b.c.) and sometimes the spermatheca (st) of the female (these are her sperm storage organs).
We have established that species in each group can be discriminated by morphologies associated with their accessory penes and its effectiveness for sperm displacement. (see Robinson & Novak 1997, Biol. J. Linn. Soc.)
Note in the following line drawings that the penes of monandrous species terminate in 2 flagella that are short and thick relative to the same structures on Group II species. Also, notice that Group III penes are quite dissimilar to those from Group II.
|Polyandrous non-tandem guarding|
|Polyandrous tandem guarding|
Electron micrographs are provided for a flagellum tip from each of 15 ischnuran species. a-f are Group II species, g-m are Group I species, and n,o are Group III species. Note tendency for monandrous species to have a little microspination on the tips of their flagella.
|I. ramburii||I. cervula|
|I. graellsii||I. damula|
|I. pumilio||I. elegans|
|I. demorsa||I. perparva|
|I. hastata||I. kellicotti|
|I. posita||I. aurora|
|I. verticalis||I. gemina|
Copulation serves: group I species only for sperm deposition, Group II species for sperm displacement and deposition, and Grouping III species for sperm displacement, deposition and in copula guarding. Notice that the penes of all species except for members of Group III in the above line drawings have paired basal spines.
Group I females mate only once, therefore with only one male. This represents a probable precursor to non-promiscuous mating systems. I am trying to define which demographic and ecological forces select for this mating strategy. I first must establish how frequently this has evolved. To do this we are sequencing the DNA from different species in the genus to determine their evolutionary relationships. We are also studying the ecology and mating behaviors of ischnuran species that previously have not been studied.
I am also interested in the functional morphology of larval damselflies and dragonflies. Damselfly larvae have 3 lamellae at the tip of their abdomen that serve in locomotion, gas exchange and as sacrificial units when grasped by predators. We have done considerable research on the comparative uses of these external structures. (see Robinson et al 1991 Oecologia 87:1-7, Robinson et. al. 1991 Am. Midl. Nat. 125:240-245, Burnside & Robinson 1995 Zool. J. Linn. Soc.)
Dragonflies have evolved from damselfly-like ancestors but do not have these external structures. Instead, they have a specialized rectum that is lined with gills for gas exchange. The rectum also serves a locomotive function in dragonflies. By contraction of the muscles surrounding the rectum, these larvae expel water with sufficient force to jet propel them through the water. Since dragonfly larvae have quite varied morphologies we believe their shapes will influence the speed with which they can move through the water.
We are investigating the effectiveness of jet propulsion and gas exchange in dragonfly larvae to assess the adaptive value of their locomotive and gas exchange systems relative to the lamellar systems we have already studied in damselflies.