  We present a comparative study of the low-frequency eclipses of spider
(compact, irradiating binary) PSRs B1957+20 and J1816+4510. Combining these
data with those of three other eclipsing systems we study the frequency
dependence of the eclipse duration. PSRs B1957+20 and J1816+4510 have similar
orbital properties, but the companions to the pulsars have masses that differ
by an order of magnitude. A dedicated campaign to simultaneously observe the
pulsed and imaged continuum flux densities throughout the eclipses reveals many
similarities between the excess material within the two binaries, irrespective
of the companion star properties. The observations show that the pulsar fluxes
are removed from the line of sight throughout the main body of the eclipses.
For PSR J1816+4510 we present the first direct evidence of an eclipse mechanism
that transitions from one that removes the pulsar flux from the line of sight
to one that merely smears out pulsations, and claim that this is a consequence
of scattering in a tail of material flowing behind the companion. Inferred mass
loss rates from the companion stars are found to be $\dot{M}_{\text{C}} \sim
10^{-12}~M_\odot$~yr$^{-1}$ and $\dot{M}_{\text{C}} \sim 2 \times
10^{-13}~M_\odot$~yr$^{-1}$ for PSR B1957+20 and PSR J1816+4510, respectively;
seemingly too low to evaporate the stars within Hubble time. Measurements of
eclipse durations over a wide range of radio-frequencies show a significant
dependence of eclipse duration on frequency for all pulsars, with wider
eclipses at lower-frequencies. These results provide a marked improvement in
the observational constraints available for theoretical studies of the eclipse
mechanisms.
