Specific, custom contrasts in EMMEANs w/ multiple nested factor levels

Question

Here is my data frame:

rearing.temp<-c("15", "15", "15", "15", "15", "15", "15", "15", "19", "19", "19", "19", "19", "19", "19", "19")
source<-c("field", "field", "woods", "woods", "field", "field", "woods", "woods", "field", "field", "woods", "woods", "field", "field", "woods", "woods")
runway.temp<-c("40","20","40","20","40","20","40","20","40","20","40","20","40","20","40","20")
velocity<-c("2.3", "2.1", "1.9", "1.9", "2.3", "2.2", "2.3", "2.0", "1.1", "1.3", "3.2", "2.0", "2.0", "2.4", "2.2", "3.0")
snail<-data.frame(rearing.temp, source, runway.temp, velocity)

Here is my model:

mod <- lm(velocity ~ runway.temp*source*rearing.temp, data=snail)

When I do an emmeans contrast:

emmeans(mod, pairwise~runway.temp*source*rearing.temp)

I get 28 different comparisons, but I am only interested in looking at the difference between the velocity of field snails reared at 15° tested at the 40° runway temperature compared to woods snails reared at 15° tested at the 40° runway temperature. I just want to do one comparison between the snails that are both reared at the same temp, are tested at the same temp, but are sourced from different habitats. Plus, when I do all 28 comparisons it's robbing statistical power and almost everything non-signficant. How can I do a specific pairwise comparison without subsetting?

Thank you,

Ricardo