Where Are We Now? Bergmann's Rule Sensu Lato in Insects

Abstract Bergmann's rule states that individuals of a species/clade at higher altitudes or latitudes will be larger than those at lower ones. A systemic review of the known literature on inter- and intraspecific variation in insect size along latitudinal or altitudinal clines was done to see how often such clines appeared and whether they reflected classwide, species-specific, or experimentally biased tendencies. Nearly even numbers of studies showed Bergmann clines and converse-Bergmann clines, where insects get smaller as latitude/altitude increases. In fact, the majority of studies suggested no clines at all. Small ranges may have obscured certain clines, while giant ranges may have introduced artifacts. Researchers examining interspecific patterns found clines less frequently than those examining intraspecific patterns because of variation among species within the clades, which renders interspecific studies unhelpful. Bergmann's rule does not apply to hexapods with nearly the same consistency as it does to endothermic vertebrates. The validity of Bergmann's rule for any group and range of insects is highly idiosyncratic and partially depends on the study design. I conclude that studies of Bergmann's rule should focus within species and look at widespread but contiguous populations to account for all sources of variation while minimizing error.     

Bacterial and eukaryote microbiomes of mosquito habitats in dengue-endemic southern Taiwan

Mosquitoes interact with the microbiome of the waters where they oviposit in several ways. Past work suggests adult mosquitoes can detect certain microbes that stimulate oviposition. The presence or absence of certain microbes in water containers thus can attract or repel mosquito species to different containers. I hypothesized that these relationships could be detected via metagenomics. I focused on two container breeders that coexist in Southern Taiwan: the dengue vector Aedes aegypti and the less competent vector Ae. albopictus. In addition to culturing, I performed 16S and 18S rDNA metagenomics assays, the latter of which had never been applied to mosquito waters before, to identify the microbial diversity of artificial containers with and without mosquito larvae. I found no correlation between mosquito presence to any features of the containers or to their microbiomes, which instead correlated strongly with location. Microbial diversity across containers was highly variable, even within the same location, with multiple taxa only found in single containers. This variability is reasonable, because mosquito gut microbiomes are also extremely variable. The possibility remains that microbes in natural containers differ significantly from those in artificial containers, and that these differences drive Aedes preferences for human-associated containers. Broad, single-microbe experimental work is recommended to identify possible attractant or repellent microbial taxa.     

Vital staining of the stick insect digestive system identifies appendices of the midgut as novel system of excretion

The stick insects or phasmids (Phamsatodea) have a series of pyriform ampulles with long, thin filaments on the posterior end of their midgut referred to as the “appendices of the midgut.” Found only in phasmids, their function had never been determined until now. Their similarity to the Malpighian tubules, which are ubiquitous insect organs of excretion, suggested a similar function. To differentiate between the appendices and the Malpighian tubules and compare functional differences between the two tissue types, vital staining (the injection of histological stains into living organisms) was done in conjunction with light and scanning electron microscopy in multiple phasmid species. The results showed that the appendices originated in the basal phasmids (Timematidae) and grew more numerous in derived species. The appendices stain selectively, notably failing to pick up the indicators of the two known systems of invertebrate excretory function, indigo carmine and ammonium carmine. Appendices sequester stains in the ampule portion before eliminating the compounds into the midgut. We conclude by confirming that the appendices do have an excretory function, but one unlike any other known in invertebrates. Their function is likely cation excretion, playing a role in calcium regulation and/or organic alkaloid sequestration. The appendices must thus be considered distinct organs from the Malpighian tubules. J. Morphol. 275:623–633, 2014. © 2013 Wiley Periodicals, Inc.