a lizard
蜥 centers on a specific animal: a lizard. It is a zoological term with no broad abstract extensions.
蜥 combines 虫 (insect/reptile) with 析, which likely contributes the sound. The character was created to represent a lizard, a reptile associated with the insect radical.
The insect radical 虫 on the left suggests a small crawling creature, and 析 on the right looks like an axe splitting wood. Picture a lizard darting out from a split log: 蜥 is a lizard.
For シャク, imagine a lizard basking on a shack in the sun: shack -> シャク, and the lizard warms itself on the wooden shack.
lizard
Komodo dragon (Varanus komodoensis)
don't judge a book by its cover
Gila monster (Heloderma suspectum)
Yemen monitor (Varanus yemenensis)
savanna monitor (Varanus exanthematicus, species of carnivorous monitor lizard native to sub-Saharan Africa); Bosc's monitor
Nile monitor (Varanus niloticus); water leguaan
Iberian worm lizard (Blanus cinereus); European worm lizard
lace monitor (Varanus varius, species of carnivorous monitor lizard found in eastern Australia); lace goanna
black-headed monitor (Varanus tristis); freckled monitor
tuatara (primitive lizardlike reptile endemic to New Zealand)
Ryukyu tree lizard (Japalura polygonata)
ridge-tailed monitor (Varanus acanthurus); spiny-tailed monitor
heloderm (any venomous lizard of genus Heloderma, incl. the gila monster and the beaded lizard)
Salamander (poem by Octavio Paz)
emerald tree monitor (Varanus prasinus); green tree monitor
worm lizard; amphisbaenian
horned lizard; horned toad
legless lizard (esp. Anguidae spp., incl. glass lizards and slow worms)
girdle-tailed lizard