Spiders construct egg sacs to protect their eggs from predators and maintain an appropriate internal environment for development. Egg sacs are produced using a variety of silk types. The outermost layer is often composed of tubuliform silk, which is unique to egg sacs and is only produced by mature female spiders. Tubuliform silk has a distinct amino acid composition and mechanical properties compared to major ampullate silk. By comparing the wettability, via contact angle measurement, of tubuliform and major ampullate silk with known amino acid sequence composition, we test the hypothesis that tubuliform silk may be specialized to maintain internal conditions of egg sacs, particularly by excluding bulk water from rain or dew from entering the sac and drowning the eggs. We assessed the variation of surface hydrophobicity of the egg sacs and major ampullate silk samples of Argiope trifasciata, Latrodectus hesperus, Larinioides cornutus, and Parasteatoda tepidariorum by contact angle determination to test the prediction that the outer surfaces of egg sacs would be more hydrophobic. In addition, we selected species with available amino acid sequences for both major ampullate spidroin (MaSp) and tubuliform spidroin (TuSp) proteins for sequence comparison, and compared the density of hydrophilic and hydrophobic amino acids to test the prediction that tubuliform silks contain more hydrophobic domains. Our findings play a key role in determining the underlying mechanisms by which 1) egg sacs maintain appropriate conditions for development and 2) tubuliform silk may have become specialized from more ancestral silk types.