Understanding growth in insects: Dyar’s Law revisited

Recently, I had a conversation with Dr. Jorge Santiago-Blay, an entomologist whose research includes fossilized insects.  A Resident Research Associate in the Department of Paleobiology with the National Museum of Natural History, and faculty member at both the University of Maryland University College and Penn State University York, Dr. Santiago-Blay’s keen interest in insects is contagious.

In his recent investigations, a small, fossilized head in amber was proving difficult to identify.  “I was working with students on this old (approximately 100 million years old) fossil, but its identity kept eluding us. What was it? We always seemed to be asking,” Dr. Santiago-Blay said.  “Eventually, it became apparent that the fossil specimen was the head of a larva. Sadly, the rest of the body was not available in the fossil. The team began to wonder how large the fossilized larva was. Because, as far as we know, there is only one such fossilized larva available, we decided to measure modern look alikes of the fossilized larva to try to guess the size of the fossilized specimen.” As Dr. Santiago-Blay’s team began garnering and analyzing data, new questions appeared. “As I dug further into the data, the available photos, and so on … I noticed a pattern that reminded me of Dyar’s Rule, but it was different because we were measuring the dimension of a softer body part, the abdomen.”

At the turn of the twentieth century, entomologist Harrison Gray Dyar, Jr. (1886 - 1929) and others researched growth patterns of Lepidoptera (moths and butterflies), Diptera (flies, such as mosquitoes), and other arthropods. Dyar identified a geometric pattern of increase in the exoskeleton length and width relationships over progressive molting events—indicating that this progression could be measured so consistently that one could reliably predict the size at which the next molting events would occur. We see the first mention of a “Dyar’s Law” in the scientific literature, referring to this progression, as early as the 1920s.

No data set is perfect; there is always extraneous data. When a scientific standard like Dyar’s Law is developed, the analyses used to arrive at the standard include filtering out the “noise” in the data. Dr. Santiago-Blay’s team is taking a fresh look at Dyar’s original, unfiltered observations for clues that might explain how Dyar reached his conclusions. 

Santiago-Blay and his team hope to find some answers in Dyar’s original field observations and his personal papers located at the Smithsonian Institution Archives. That will require transcribing a number of Dyar’s field books cover to cover to make as thorough an examination as possible. Last spring, the Smithsonian Transcription Center held a #DigIntoDyar campaign to tackle a number of Dyar’s sawfly field books from the Archives collections.

“I think the Transcription Center and its volunteers are wonderful. The work they do is extremely important. Hopefully, their help transcribing Dyar’s materials will make it possible to discover more answers and better understand Dyar’s Law,” Santiago-Blay said. More of Dyar’s field books are available now for transcription at the Smithsonian Transcription Center. 

Related Resources

• Record Unit 7101. Harrison Gray Dyar Papers, 1882-1927, Smithsonian Institution Archives
• Harrison G. Dyar Notebooks, Department of Entomology, 1882 - 1925, Smithsonian Institution Archives
• The Impossible Case of Harrison G. Dyar, The Bigger Picture, Smithsonian Institution Archives
• The Bizarre Tale of the Tunnels, Trysts and Taxa of a Smithsonian Entomologist, Smithsonian Magazine