The apparent disconnect between the spidroin expression patterns and the amino acid compositions in the silks of most species might lead us to conclude that: (1) the silk nanostructures and mechanical properties of different spiders do not respond similarly to variations in spidroin expression, and (2) the relative contributions of change in spidroin expression in inducing spider silk mechanical property variation is minimal at best. We, nevertheless, presume caution before drawing definitive conclusions about the influence of MaSp expression in light of the following. Firstly, in our RT-PCR reactions any specific amplification of MaSp paralogs could arise in any given species using a single pair of primers . This may cause falsely elevate expression levels of certain MaSp transcripts. Examination of the melt curves for the genes screened suggested that in some species there were indeed multiple amplifications. The most discernible example of this is found in the melt curves for MaSp1a and MaSp2a, particularly for Nephila plumipes (see S3 Fig). Secondly, the amplification of non-orthologous MaSp loci across species with single primer pairs could lead to falsely elevated or falsely lowered detection of expression levels . Any of the MaSp-targeting primers could thus have identified a single MaSp transcript in one species, a single non-orthologous transcript in a second species, and/or multiple non-orthologous transcripts in yet another species. Thirdly, since the full length MaSp sequences for each of the species under investigation herein is not known, we designed primers based on the MaSp sequences for closely related species (Argiope trifasciata and Latrodectus hesperus). The amplification efficiency thus may potentially be biased toward species with the greatest sequence homology (i.e. Argiope keyserlingi and Latrodectus hasselti). Indeed only in L. hasselti was expression of MaSp1b and MaSp2b comparable to that of MaSp1a and MaSp2a. Lastly, some degree of gDNA amplification cannot be ruled out as influencing our results. We, however, did not expect this to cause major expression value biases within any particular species.